The steel belt conveyorADVICE FOR C ALCULATION AND DESIGN

CONTENTS

Introduction ................................................... 1Material handling ........................................... 1Design criteria and rules ............................. 2Selection of belt grade ................................ 2Selection of belt type ................................... 3Selection of belt size .................................... 3Belt joint ......................................................... 3Selection of terminal drums for plain Steel belts ....................................................... 4Selection of drum profile ............................ 5Selection of terminal sheaves for True-tracking belts ........................................ 6Belt tensioning for steel conveyor belts..... 7Tensioning mechanisms ............................... 8Selection of belt supports .......................... 9Selection of safety devices and belt guides ...................................................... 12Steel belt maintenance ................................ 15Cleaning belts ................................................ 16Power calculations ........................................ 17

This handbook is an offprint from The Steel Belt Book by SandvikProcess Systems GmbH, Fellbach, Germany. Copyright© 1993.Allrights reserved.

Nothing in this handbook shall create or imply any warranty beyondor in addition to those included in our standard terms and condi-tions of sale related to the sale of our products.

1

Introduction”Conveyors” are defined as either fixed or port-able devices for moving materials between twofixed points at the same or different elevations,with continuous or intermittent forward move-ment. Many types and sizes of conveyors havebeen developed, but our focus is on steel beltconveyors.

During the first four decades of the last cen-tury, the steel belt conveyor was used to move agreat variety of bulk materials without regardfor the belt´s specific properties. Eventually, thespecific properties of the steel belt were morefully identified and put to work.

For example:

1. The dense, smooth surface of a steel belt isimportant for simple discharging ofproducts.

2. The low coefficient of friction is importantfor sliding and accumulation of products.

3. The elevated temperature properties areimportant for baking and handling hotproducts.

4. The hygienic properties, particularly forstainless steel, are important for processingand handling foods.

This publication will deal with the fundamentaltechnical rules for the proper design of steel beltconveyors and the calculations necessary forconstruction.

In this booklet applications where the mate-rial on the steel belt is subjected to a physical orchemical change, referred to as ”processingsystems” have not been covered. However, thefundamental technical rules for the steel beltconveyor also apply in these applications.

The main types of belts for belt conveyors are:1. Chain belts2. Plastic belts3. Plastic-coated fabric belts4. Rubber-coated fabric belts5. Rubber-coated wire reinforced belts6. Solid steel belts7. Wire belts

In order to decide which type of conveyor beltto use, some of the following points should beconsidered:

The first, and probably the most importantitem, is the product to be moved. For example,if the product is small, a wire belt may not besuitable. If the product is hot, rubber or plasticbelts may be eliminated. The quantity and thephysical and chemical conditions are considera-tions necessary for selection of the proper belt.The second consideration is the distance thatthe product is going to be moved. The third is the speed of movement.The fourth is loading and unloading features,and the integration of the conveyor with otherproduction equipment.The fifth is maintenance.And finally, the economic factors.

Material handlingIn engineering for material handling, con-sideration should be given to a number ofadditional factors. More efficient sorting,loading and unloading of goods increase thereliability, decrease the costs, and reduce therisk of accidents and strain on workers.

Computer-aided material handling systemsare a major factor in todays workplace. Eachworkplace has to be examined to determinewhich improvements in material handling arenecessary.

In analyzing and planning a material hand-ling system, there are a number of factors to beexamined.For example:1. Product

a. dimensionsb. weightc. shaped. fragilitye. quantity

The steel belt conveyor

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Sandvik

1000SA ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

1200SA ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

1700SA ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

1050SM ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

1150SM ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

1500SM ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

1650SM ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

1850SM ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

1100C ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

1300C ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

1320C ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

Figure 1Selection of a belt grade.

GRADE

● = fair ● ● = good ● ● ● = very good ● ● ● ● = excellent

2. Workspacea. routesb. available timec. production factors

3. Maintenance and repair4. Alternative methods

In the final analysis, all these contribute to costsavings – usually the major determining factorfor installing a steel belt conveyor.

Design criteria and rulesIn the design and selection of the variousmechanical components which make up theconveyor, the basic design rules were estab-lished many years ago by the first producer ofsteel conveyor belts, Sandvik. There has beenvery little published on steel belt and compo-nent development, and this is one of the mainreasons for this publication.

The present rules for design and calculationprovide for optimal performance of the steelbelt conveyor.

Selection of belt gradeFirst, let´s look at belt grades. Each conveyorapplication has its own unique demands. Tomeet these demands, it is essential that theproper steel belt grade be selected. Figure 1can be used as a reference in the selection

process. This figure lists critical belt propertiesand shows the relationship of these propertiesto the various belt grades.

In general, Sandvik 1200SA and 1300C aremost frequently selected for material handlingand processing conveyors. As Figure 1 indi-cates, if an application requires the belt to havehigh wear resistance and stability againstuneven temperature, Sandvik 1300C is verywell suited for the purpose.

Hot operating environments.The maximum practical temperature in whicha steel belt should operate is 350 °C. Appli-cations with higher temperatures require spe-cial considerations for selecting the belt grade.

It is important that the temperature isuniform across the width of the belt. Local hotspots can cause temporary distortion, erraticbelt tracking, and loss of load. Under severeconditions, where uneven heat cannot beavoided, special devices for belt tracking maybe required.

Sub-freezing operating environments.For applications where the steel belt is con-stantly working in low temperatures below- 20°C or less an austenitic stainless steel ispreferred, i. e. Sandvik 1200SA or 1000SA.

3

Selection of belt typeThere are three types of steel belts: Plain, rubbertrue-tracked and steel spiral true-tracked. Plainbelts are most common, but there are timeswhen the other types are better suited for theconveyor application.

For tracking, the belts can be provided withV-ropes, either rubber or in the form of aspecially designed steel spiral. If required,the product side of the belt can be fitted withretaining strips to keep the conveyed materialon the belt or with transverse flights to preventmaterial from sliding backwards when thebelt is steeply inclined. Material propertiesaccording to PS-SB-5507.

Selection of belt sizeFor piece goods, the belt width is determinedby the size of the individual objects being con-veyed, and by the way they are to be placed onthe belt. For bulk handling, a calculation mustinclude the following factors:

1. Volume

2. Distance

3. Material Characteristics

4. Incline of the Conveyor

Your local Sandvik office could be contacted fortechnical assistance. Standard dimension rangefor Sandvik steel according to PS-SB- 101.

Belt jointSteel belts for conveyors are joined by eitherwelding or riveting. Special techniques and toolshave been developed for both welding and rivet-ing, and unless someone has these skills andtools, it is advisable to have this work done bythe belt supplier. Most belts are joined at thework site; however, specialty endless belts maybe manufactured at the factory and installed atthe work site.

The belt must be carefully prepared and cutso that the two ends fit very closely togetherbefore joining.

A special fixture holds the two ends in posi-tion as an automatic welding head makes theweld. Depending on the belt grade, the weld iseither cold worked or heat treated to restoremost of the original mechanical properties, andthe surface of the weld is finished to the desiredsmoothness. Figure 2 shows fixture for automa-tic welding of belt joint.

Riveted belts have two types of joints: over-lap and butt strap, shown in Figure 3.

The overlapped riveted joint is the easiest tomake and can be performed by factory trainedmaintenance personnel. If a smooth workingsurface is required, the butt strap joint withflush rivet heads should be used. A butt strapjoint is also used on reversible conveyors.

4

Selection of terminaldrums for plain steel beltsIn designing a steel belt conveyor, the anticip-ated fatigue life of the belt and the belt joint isan important consideration. Preventing prema-ture fatigue failure is the basic objective ofhaving the proper drum diameter.

For a plain belt with riveted joint, a maxi-mum belt tensile stress of 25 N/mm2 is recom-mended, and for a plain belt with a welded jointwe recommend a maximum of 50 N/mm2.Tensile stress is defined on page 6. Under suchconditions, determine the approximate drumdiameter by use of the diagram in Figure 4.

For a welded joint, the diameters given havea safety margin where fatigue should not occurwhen the belt is operating under normal condi-tions and where no corrosion, abrasion, orimpact are expected. Usually, riveted joints haveto be replaced within 10–22 months.

The diagram in Figure 4 is valid for tensilestresses up to 25 N/mm2. For tensile stressesbetween 25 and 50 N/mm2, the drum diametershould be increased by 10 %. This figure isvalid for Sandvik 1200SA and 1300C steel beltgrades. Diagrams for other steel belt grades areavailable.

Figure 2Welding fixture.

Min. drum diameter = K x belt thickness

S = 1, 2 or 3 shifts/day

V = belt speed in m/min

L = centre distance in m

Figure 4Determination of drum diameter.

V

L x S

Figure 3Overlap and butt strap joints.

5

Case 1 is valid for a tensile stresss

p ≤ 25 N/mm2

Case 2 is valid for a tensile stressσ

p > 25 N/mm2

up to ≤ 50 N/mm2

Also valid for belts working above 100 °C.

σp = Tension force in belt, N

Belt width, B x Belt thickness, t, mm

Case 1 2

b b

B c min c min

≥ 200

0.5 B c + 60 B - 60 B - 20

≤ 500

≥ 600

0.5 B c + 100 B - 150 B - 50

≤ 800

≥ 1000

B - 400 c + 100 B - 200 B - 100

≤ 4500

Figure 5Drum profile. f = b - c

2

Figure 6Drum dimensions.

Dimensions b, c, and f may vary, depending onbelt size and belt operating conditions. SeeFigure 6.

Selection of drum profileThe drum profile strongly affects the guidanceof the belt. Generally, the overall drum widthshould be narrower than the belt width. Thecentral portion of the drum face should becylindrical, and the portions on each sideshould have a slight taper. See Figure 5.

For special cases, where higher belt stressesare involved, other rules may apply and the beltmanufacturer should be consulted.

The tolerance of the drum should be meas-ured as a peak-to-valley measurement whilerotating the drum on a shaft. This is usuallydone with a dial indicator which is movedtransversely across the cylindrical face of therotating drum. The peak-to-valley reading,depending on the width of the cylindrical por-tion, should not exeed 1.25 x 10-4 x width.This tolerance does not apply to the taperedportions.

The drum should be designed to withstand amaximum deflection of 1:2000 measured be-tween bearing centres.

The conveyor drums are generally made offabricated steel or cast iron. However, for bet-ter friction, the drum face can be lagged withrubber or other high friction materials. Underwet conditions, provision should be made forliquid removal by means of holes, grooves, or aherringbone pattern in order to maintain beltcontact with the drum surface.

B

c ff

Cylindrical portionb

Drum widthf = tapered portion

D

Drumdiam

f100Belt width

6

Before calculating the number of sheavesrequired, the max pull of each sheave has to bechecked. See ”Minimum belt tension” page 7.

The belt stress at the terminal sheaves iscomposed of three components:

1. Tensile stress, σσp, due to the pretension andpulling force, (see page 5).

Normally 15 N/mm2 is recommended as themax. tensile stress for a true-tracking belt.

2. Bending stress, σσb, defined as E x tD

3. Additional stress, according toThimoshenko´s ”Theory of Plates andShells.”

The maximum permissible belt stress at theterminal sheaves is 400 N/mm2.

Selection of terminal sheavesfor true-tracking beltsFor true-tracking belt conveyors in light dutyapplications, the arrangement at the terminalends is quite simple. Narrow belts with a singleguiding strip can generally be supported by onetrue-tracking sheave with a groove. The diame-ter of the sheave should be determined accord-ing to the same rules as described earlier forplain belts. For wider belts and belts with twoguiding strips, support sheaves are normallyrequired.

When using more than one sheave on thesame shaft, the sheave diameter tolerance is thesame as the drum diameter tolerance for plainbelts. The number of support sheaves dependson belt width, stress, and the requirements offeeding and discharging. See Figure 7.

Sheaves for true-tracking belts with rubberguide strips (EURO standard) should have agroove profile as shown in Figure 9.

Figure 7Calculation of the number of required sheaves.

Referring to Figure 7, the maximum beltsection, L, supported by one true-tracking orsupport sheave can be calculated by using thefollowing formula:

400 - σσ b - σσ p E x t 2

Lmax = x 1.1 x σσ p σσb

D is the diameter of the sheaves (mm); E is themodulus of elasticity (N/mm2); t is the beltthickness (mm); σσb is the bending stress(N/mm2); and σσp is the tensile stress (N/mm2).The resulting value of L max should not beallowed to exceed 400 mm.

As an alternative on belts with more than oneguiding strip, sheaves with chamfered sidesinstead of grooves can be used, see Figure 8. Insuch a case contact should be made with thesteel belt manufacturer, for further advice.

Figure 8Sheave or drum with chamfered sides.

Figure 9Groove dimensions for rubber true-tracking guide strips.

10 mm

20.3

17

4

38°

B

L LL

Belt width

V-r

ope

She

ave

Sup

port

Sup

port

She

ave

She

ave

V-r

ope

She

ave

drum3 mm sheave

7

Belt tensioning forsteel conveyor beltsTension on a steel belt conveyor is generallyapplied against one of the terminal ends, calledthe ”tail” drum or sheave.

Because the steel belt will not elongate innormal conveyor usage, the tensioning arrange-ment is fairly simple. However, at elevated tem-peratures, consideration has to be given to ther-mal expansion.

There are four main reasons for tensioning abelt:1. To obtain enough friction force between the

belt and drive drum/sheave for running thebelt.

2. To obtain enough pressure between the beltand terminal drums for guidance of plainbelts.

3. To obtain optimum belt flatness.4. To obtain proper belt tension for operational

requirements.

Minimum belt tensionWhen the required pull, PT (see page 17), to runthe belt has been determined, it is necessary tocheck that the belt tension, Qp, is sufficient toobtain enough friction between the belt anddrum.

For a plain belt and drum arrangement, thefollowing formula applies:Minimum Qp = 3.3 PT for a steel drum or 1.67 PT for arubber lagged drum.

Figure 10Groove dimensions for steel-spiral true-tracking guide strips.

However, the normal pre-tension stress inplain belts should be no less than 10 N/mm2.The total min. tension, Q, is thus arrived at asfollows:Q = 2 x 10 x B x t

where B is the belt width (mm) and t is the beltthickness (mm).The larger, Q or Qp, should be chosen, as totalpre-tension for the plain belt.

For true-tracking belts, other conditions applywith the following assumptions being made: amax pull of 2500 N is assumed for each true-tracking sheave, and a max. pull of 1250 N foreach support sheave, to obtain enough friction.

For true-tracking belts, normally a pre-tensionstress of 7 N/mm2 is recommended for true-tracking belts. The normal pre-tension for atrue-tracking belt is thus arrived at as follows:Q = 2 x 7 x B x t

Maximum belt tensionBelt tensions must not be so high, that themaximum tensile stresses – as mentioned onpage 5 (plain belts) and page 6 (true-trackingbelts) – are exceeded!

Sheaves for true-tracking belts with steel-spiral guide strips should have a groove profileas shown in Figure 10.

31

6 2624

60°

min 70

8

Tensioning mechanismsIt is imperative that the components in the ten-sioning mechanism have sufficient rigidity toprovide parallel movement for the two terminalshaft bearings. At least one of the tension shaftbearings should be adjustable for making con-veyor and belt tracking corrections. The termi-nal framework and interconnecting membersshould also have proper strength and rigidity.

The tensioning arrangement can be designedin many different ways, depending on the ope-rating conditions of the conveyor and the sizeof the belt. When a new belt is installed, thedrum should be positioned near the outermostpoint in order to provide space for shorteningwhen a future remaking of the joint is required.

An exception to this is where the belt will beexposed to an elevated temperature. In suchcases, the thermal expansion has to be con-sidered since the steel belt will expand.

Manual belt tensioning

The simplest device for manually adjusting belttension consists of two take-up bearings, seeFigure 11.

This type is used primarily on conveyorinstallations with centre-to-centre distances ofup to 30 m, operating at room temperature. Foradjusting small true-tracking belts, simple pil-low blocks with a means of adjustment alongthe conveyor length axis can be used.

2.Tension frame and coil springs.

This method involves the placement of coilsprings between the moving tension frame andthe conveyor structure. This design is morecompact than the counterweight method.Because the spring pressure varies with thedegree of compression, special care must betaken in selecting the spring shape and prop-erties. Hydraulic or pneumatic cylinders withsuitable pressure controls can be used instead ofthe springs. These are particularly advantageouswhere the tension requires frequent adjust-ments, see Figure 13.

Automatic belt tensioningAn automatic tensioning device should be usedon conveyor installations where the centre-to-centre distance is greater than 30 m, or the

Figure 11 Belt tensioning by means of take-up bearings.

operating temperature is either lower or higherthan the ambient temperature. Automatictensioning can be achieved in many differentways. Three methods will be illustrated:

1.Tension frame and counterweight.

This method involves the mounting of the taildrum assembly on a moving tension framewhich is actuated by a counterweight, seeFigure 12.

Figure 12Belt tension by means of a tension frame and a counterweight.

Figure 13Belt tension by means of a tension frame and compression springs.

9

3.Torsion shaft.

This method is normally used for wide beltswhere a tension frame is not practical. When thewidth of the frame exceeds its length, this cancause structural instability and deflection prob-lems. In such cases, the springs or cylinders canbe made to act on a torsion shaft which does notdepend on the tension frame for the transmittalof tension forces to the bearings. In the torsionshaft arrangement, the adjustable drum shaftbearings are actuated by threaded rods whichprovide individual adjustment of the bearings.See Figure 14.

NOTE 1: In determining the weight of the belt strand, the weight of the true-track strip, if applicable, should be included. The weight ofone true-tracking rubber strip is approximately0.5 kg/m; and, of one true-tracking spiral,0.25 kg/m.

NOTE 2: The density of the steel belt material isapproximately 7.9 g/cm3. See our data sheets fordetailed information on density of steel beltgrades.

Selection of belt supportsThe steel belt can be supported between the ter-minal ends by either slide supports or idlers.Slide supports are commonly used when theconveyor is short and the load is not extremelyheavy or abrasive. Idlers with low friction bear-ings minimize the pull and wear on the belt.

Slide supportsCarbon steel belts slide easily over metal, wood, or plastic, while stainless steel belts slide easilyover wood or plastic. Stainless steel belts shouldnot be used to slide over mild steel, stainless steelor aluminium slide supports.

IdlersAn idler can be either individual wheels on acommon shaft or a tubular roller. See Figure 16.

Figure 14Belt tension by means of a torsion shaft arrangement.

Belt tension formulaActual belt tension, S, in one belt strand can be approximated by checking the catenary, or beltsag, f, on the return strand between two beltsupport rollers. See Figure 15.

Figure 15 Catenary, or belt sag.

The following formula applies:

5 q • a2

f = • •103

4 S

where S is the belt tension (N); q is the weightof belt strand (kg/m); a is the distance between two rollers (m); and f is sag in mm.

Figure 16Idlers a) wheels on a common shaft Idlers b) tubular roller

a

f

a)

b)

10

Generally, the length of the support idlersshould be equal to the belt width minus 100 mm.Idlers should be reasonably balanced.

For noise reduction at high belt speeds andfor increased friction for improving rotation,rubber rings may be fitted to idler rolls. Rubberrings with elliptical cross-sections to supportthe return strand are preferred when stickymaterials are handled.

Break-point unitsIdlers are also used where a belt must change its direction of travel from the horizontal plane.For example, when the return strand must be“caught-up” to clear obstructions. The changeof direction should not be more than twodegrees per single idler. When greater bends arerequired, a cluster of closely spaced idlers canbe used, called a break-point unit. In such cases,a bend of two degrees maximum per idlerapplies to the first and last idler in the cluster; abend of three degrees maximum applies to theintermediate idlers, see Figure 17.

Figure 17Break-point unit (cluster of idlers with respective bending angles).

The break-point unit should be able to pivot.It has a considerable effect on the guidance ofthe belt and, therefore, its pivoting bearingsshould be adjustable both horizontally and ver-tically. The length of the idlers in the break-point unit should be 50 mm narrower than thebelt width.

Idler spacing in the carrying strandIn determining the distance between support idlers in the carrying strand, the objective is toavoid high bending stresses. For a belt operatingunder normal conditions, the catenary, or sag,between two idlers should be limited to 25 mm.The nomograph in Figure 18 can be used toobtain the idler spacing. The following formulaapplies:

4 f Sa = • •

5 103 (q+qg)

where f is the sag of belt (mm); q is the weightof the belt (kg/m); qg is the weight of the load(kg/m) on the belt; a is the distance betweenidlers (m); and S is the tension in one belt strand(N).Depending on the type of process or theproduct being conveyed the belt sag mighthave to be less than the above calculatedmaximum value.

3°

3°2°

2°

11

Figure 18Nomograph for idler spacing.

Load

on

belt

in k

g pe

r lin

. met

er

0305

0610

0910

1220

1520

1830

2130

2440

Example:Load on belt 35 kg/m

Belt 600 x 1.0 mmweight 4.5 kg/m

Answer: 1700 mm idler spacing

89

82

74

67

60

56

45

37

30

22

15

7

kg/m

15.0

12.0

9.0

7.5

6.0

4.5

3.0

1.5

▼ ▼ ▼ ▼

▼

▼

▼

Idler spacing in the return strandIn the return belt strand, the distance between the tension drum and the first idler should notbe more than 3 m. For plain belts, a distance ofup to 8 m between idlers can be used if there is

sufficient clearance for the sag. For true-tracking belts, the distance can be calculatedusing the weight of the true-track strips as qg.See page 10.

Weight of steel belts

01.5

03.0

04.5

06.0

07.5

09.0

12.0

15.0

Idler spacingmm

12

Selection of safetydevices and belt guidesIn the design of a steel belt conveyor, certain safety devices and belt guides have to be included. These can be divided into five groups:1. Safety scrapers2. Drum cleaners3. Belt cleaners4. Belt guides5. Limit switches

Safety scrapersThe safety scraper prevents foreign matter from getting between the belt and the tail drum,thereby preventing belt damage which couldlead to belt failure. The safety scraper for plainbelts is made in the shape of a plough that rideson the return strand just before the drum, seeFigure 19.

Drum cleanerThe drum cleaner prevents foreign matter from accumulating on the surface of the tail drumwhich, in case of build-up, can cause the dimen-sion tolerances of the drum to change. Thisresults in erratic belt tracking. Any materialremoved by the drum cleaner or groove scra-pers should be prevented from falling betweenthe drum and safety scraper, see Figure 21.

For a fabricated steel drum, the drum cleaner blade should be made from plastic or carbonsteel belt material. If the drum has trackinggrooves, groove scrapers should also be used.

Figure 19Safety scraper for plain belts.

It is important that the safety scraper floatson the belt and provides full contact. For car-bon steel belts, the wear edge should be madeof belt material; for stainless steel belts, thewear edge should be made of a softer material,such as plastic, rubber, or wood.

If the safety scraper is wider than the belt,the wear edge extending outside of the beltwidth must be made of a softer material thanthe belt. This will help avoid belt edge damagewhere the belt edges contact the scraper.

The safety scraper for a true-tracking belthas notches in the wear edges to accommodatethe guiding strips, see Figure 20. The materialadjacent to the true-tracking guide stripsshould be a soft material, such as rubber.

Figure 20Safety scraper for true-tracking belts.

Figure 21Drum cleaner arrangement.

13

Belt cleanerThe belt cleaner removes residual material from the product side of the belt, at the dischargelocation. If not removed, residual material cancause build-up on the return strand idlers,causing erratic belt tracking. The most commonbelt cleaner is the tangential type shown inFigure 22.

Belt cleaners should not harm the belt in anyway. Therefore, in the proper choice of thematerial for the cleaner blade, the steel beltgrade, the product handled, and the temperatureof the belt and product should be considered.

A general rule is that the scraper blade for a carbon steel belt should be as hard as the belt.The scraper blade for a stainless belt should besofter than the belt.

For carbon steel belts, scraper blades are nor-mally made of hardened and tempered carbonsteel. For stainless steel belts, scraper blades arenormally made of brass, polyamide, PVC, orother reinforced plastic material.

Relatively hard scraper blades should be used when handling abrasive products to prevent thehard particles from becoming embedded in theblade.

The scraper blade should not be pressed har-der than necessary against the belt. Undue pres-sure can cause scratches that may deform andharm the belt. An average pressure of 100 N/mof scraper width for tangential cleaners givessufficient cleaning pressure. For radial type beltcleaners, double the above pressure.

The belt cleaner should be designed so thatthe scraper blade does not apply pressure on thebelt edges. For plain belts, a scraper blade madeof metal should be 30–50 mm narrower than thebelt width. For true-tracking belts, which haveless lateral movement, the blade width can be6 mm narrower than the belt width. Whenapplying a belt cleaner to a belt with attachedretaining strips on the product side, there areother considerations, and these should bediscussed with the belt manufacturer.

Belt guiding devicesTheoretically, a steel belt operating under nor-mal conditions, should center itself on the ter-minal drums or sheaves of a properly designedconveyor. However, it has been found that theterminal guiding effect is limited to a distance ofabout ten times the belt width. Therefore, aconveyor which falls outside of this ratio, needssome type of guiding devices. These can beeither static devices which forcibly limit thelateral movement, or active devices which canincrease the force needed to correct the lateralmovement. In either case, the devices must notharm the belt edges.

Either static or active belt guides should belocated in the top and bottom strands at adistance from both terminals of ten times the

Figure 23Angle of tangential belt cleaner.

Figure 22 Tangential belt cleaner.

The recommended scraping angle of a tan-gential belt cleaner is thirty degrees from thetangent as shown in Figure 23.

Figure 24Radial type belt cleaner.

A radial belt cleaner, as shown in Figure 24, should be used for a steel belt with a butt strapjoint. This type of belt cleaner should also beused when the belt direction is occasionallyreversed, or when the scraper blade is made of asoft material, such as rubber or felt.

drum

30°

14

belt width. For longer conveyors, additionalguides should be located at intervals of 15–20 m.

Keep in mind that all steel belts have a certain deviation from a theoretical straight line.Therefore, it is a good rule to provide sufficientclearance for the belt edges in the conveyorframework, tensioning devices, belt guides, etc.A clearance of 100 mm for each belt edge issuggested for plain belts and 25 mm for true-tracking belts.

When closer belt guiding is required, anautomatic belt guiding device should be used.

Static belt guides are often made of wood, brass,or plastic, in the shape of bars with a minimumlength of 500 mm. The height of the bars shouldexceed the anticipated sag of the belt betweensupport idlers, see Figure 25. An alternative tothe bars is the use of stationary rollers made ofhardened steel.

Figure 25Static belt guide.

Active belt guides are an arrangement of rollerspositioned on a spring-loaded frame, see Figure26. These rollers should have a minimum dia-meter of 50 mm for belt speeds up to 90 m/min,and approximately 150 mm for higher speeds.

Figure 26Active belt guide with pairs of spring-loaded rollers.

The springs should have a pre-tension of100–150 N. Surface hardened rollers are recom-mended.

Another active guiding device is theadjustment of tension on one side of the belt.If the steel belt moves to one side due to adisturbance from the operation, the tension canbe increased on the side towards which the beltmoved. The steel belt moves to the side of thelowest tension. By adjusting the terminalbearings, the belt position can be corrected.Usually, only small adjustments are required onthe terminal bearings, because the belt is verysensitive to changes in tension.

The movement of the belt can be kept underclose surveillance or monitored by the use of asensor such as a limit switch. The limit switchactivates a signal, such as a light or horn, toalert the operator to make manual corrections.

If frequent tracking disturbances are antici-pated, or if only minimum lateral belt move-ment is permitted, an automatic belt trackingdevice should be used. In automatic systems,the bearing adjustment is accomplished by let-ting the signal from the limit switch be trans-mitted to an electric control unit, which in turnactivates an electric drive unit which moves oneof the terminal bearings, see Figure 27. As analternative, the sensor can be a pneumatic orhydraulic gauge which in turn acts on a pneu-matic or hydraulic cylinder attached to the ter-minal bearing.

Figure 27Automatic belt tracking device.

M

15

Steel belt maintenanceThe service life of a steel belt depends not onlyon correct design, installation, and operatingconditions, but also on proper belt main-tenance. Periodic inspections of the belt and themechanical parts that come in contact with thebelt are recommended. Some of the more com-mon items to inspect are as follows:

Deformed or burred edge.

The belt should be closely inspected to makesure it has not come in contact with any con-veyor structures, see Figure 28. Such contactcan cause belt edge deformation and burrs. Theburrs should be removed by filing.

Temporary waves and blisters.

Uneven temperatures across the belt can causewaves or blisters to appear in the steel belt.These normally disappear when the belt returnsto a uniform temperature condition.

Loss of flatness.If the entire belt loses its flatness, accumulatedforeign matter on drums or belt supports maybe the cause. Heavy scratching can also causebelt deformations. Worn cleaner blades cancause surface scratches. Careless loading of anabrasive product which slides when first loadedcan cause surface scratches. Where a productimpacts the belt on loading, particularly a stain-less belt, the belt may deform.

In many cases, the belt can be flattened bytrained service people using special serviceequipment.

Wear on riveted joints.

Normal wear on a riveted joined belt may causethe rivets to loosen. The loose rivets should beremoved and replaced with new ones. If neces-sary, remake the whole joint.

Cracks on welded joints.

Hairline cracks on a welded joint are normally asign of fatigue. The weld should be repaired orremade.

Cracks in the belt.Hairline cracks in the belt are also normally asign of fatigue. If the cracks appear at the beltedge, they may be repaired by welding, or bycutting away a crescent shaped piece and filingthe edges round, see Figure 30.

Figure 28 Proper belt edge.

File the belt egdes round

Figure 29 Removing minor deformations by cutting.

CorrectWrong

SectionA – A

A

A

Edge waves and deformations.

In severe cases of improper tracking or unevenpressure from the belt cleaner or scraper,local edge waves may occur in the belt. It isimportant to remove these deformations.Removing the deformations by cutting isillustrated in Figure 29.

Figure 30 Small edge crack removed by cutting.

If a crack develops from the belt edge andbecomes too long for belt edge trimming, andwelding is not available for repair, a small holecan be drilled at the end of the crack to preventfurther spreading, see Figure 31. Thoroughlyremove burrs at the drilled holes.

16

If a crack appears that is not at the belt edge, small holes may be drilled at each end of thecrack to prevent spreading, see Figure 32.

Figure 32 Crack stopped by drilled holes at each end.

Figure 31 Stopping a crack by “stop drilling”.

Cleaning beltsPeriodically, it is advisable to clean a steel belt thoroughly.

The food industry often uses hot water andsteam to clean steel belts at routine time inter-vals. When applying heat, if the belt is running,apply the heat uniformly across the entire widthto prevent mistracking. True-tracking belts andbelts with bonded retaining strips should not beheated above the max recommendedtemperature for the bond.

After a carbon steel belt has been cleaned, it should be greased with an acid-free oil to pre-vent rusting, unless it is immediately put intooperation.

The measures described to handle minorfatigue cracks by “stop drilling” have to beconsidered temporary. If the damage is toolarge, the best solution may be to splice in anew belt section. The length of such a sectionshould not be shorter than 2/3 of the terminaldrum/sheave circumference to prevent twojoints from being on the terminal simultaneous-ly.

Rollers.

Check all rollers and other rotating parts tomake sure they move freely and that wear-partsare not unduly worn.

17

P4 = k2 x qgwhere k2 is 7 for belt speeds under 60 m/minand 9 for belt speeds above 60 m/min. and qg isthe product load (kg/linear m) on the belt.

P5 = qg (15Bm + 2)

where qg is the product load (kg/linear m) onthe belt and Bm is the belt width (m). For sticky products, this value should be doubled.

P6 = 0.01xQ

where Q is the tension force (N) related to belt tension, generally 20xBxt (see page 8) where B is the belt width (mm) and t is the belt thick-ness (mm). The coefficient 0.01 applies only toroller bearings or ball bearings.

P7 = 500xBm

where Bm is the belt width (m).

P8 = 10xqgxh

where qg is the product load (kg/linear m) onthe belt and h is the height lifted (m). The valueis negative for a declining conveyor.

P9 = k3 x L 4

where k3 is 0.5 when the side skirts have thefunction of preventing material falling off thebelt and normally do not touch the material, is1.0 when the side skirts touch the material, andis 2.0 if the material is sticky. L4 is the productload on the belt (kg/linear m) times the lengthof side skirts (m).

The total pull, PT, is the sum of P1 to P9. It is suggested that a safety factor of 1.2 be appliedto the total pull.

The required effect, E, in kilowatts (kW) onthe drive terminal shaft can thus be calculated asfollows:

E =1.2 PT x V

60000

where PT is the total pull (N) and V is the belt speed (m/min).

The effect, E, arrived at is thus the net effecton the terminal shaft. This value has to beadjusted by the various service factors whichapply to the selected drive unit and transmis-sion. Consideration must also be given to thecapacity of the drive unit to withstand the initialresistance which occurs during start-up of beltswith heavy loads.

Power calculationsOnce the steel belt conveyor application isknown, the driving power requirements (pull)can be calculated. The pull necessary to over-come friction is generally made up of thefollowing:

P1 = pull required to overcome friction in support rollers.

P2 = pull required to overcome the extra resist-ance when the belt is supported by rubber-lagged rollers.

P3 = pull required to overcome friction from slide supports.

P4 = pull required to accelerate the load.

P5 = pull required to overcome friction at scra-pers when discharging the load.

P6 = pull required to overcome friction in ter-minal bearings.

P7 = pull required to overcome friction at safetydevices.

P8 = pull required to lift the load on inclined conveyors. This can be a negative value.

P9 = pull required to overcome friction of side skirts.

The components listed above can be estimated with sufficient accuracy, using the followingformulas which are expressed in Newtons (N).

The variables in the formulas below are expres-sed in kilograms (kg). The conversion from kgto N have been made in the constants.

P1 = 0.1 [L1 + W1 + (z x r)]

where L1 is the load on belt (kg/m) times therolling length with load (m); W1 is the weight ofbelt (kg/m) times the total rolling length (m); zis the number of rollers; and r is the weight (kg)of the rotating part of each roller.

P2 = 0.16 (L2 + W2)

where L2 is the load on belt (kg/m) times therolling length with load (m) on rubber rollersand W2 is the weight of belt (kg/m) times thetotal rolling length (m) on rubber rollers.P3 = k1 (L3 + W3)

where k1 is 4 for steel, metal, or plastic supportsand 6 for wood supports; L3 is the load on belt(kg/m) times the sliding length with load (m);and W3 is the belt weight (kg/m) times the totalsliding length (m).

18

Typical steel belt conveyor calculation (example):

Data: Sorting conveyor.

c-c distance: 150 mLoad: 60 kg/mSpeed: 130 m/minDimensions: 1000 x 1.2 mmGrade: 1300CShifts: Two per day

Weight of belt (kg/m):

Density: 7.85 g/cm3

(1000x1000x1.2x7.85)/(103x1000) = 9.42 kg/m

Idler spacing: (top strand)

See nomograph (fig 18):

Weight of belt 9.42 kg/mLoad on belt 60 kg/m 1850 mm idler spacing.Note that maximum parcel weight has not beenconsidered.

Number of idlers: 150000/1850 = 81

Number of rollers in return strand: on page 12maximum recommended distance betweenrollers is 8 m. But between tension drum andfirst roller the distance should be no more than3 meters. That gives (150-3)/8+1= 20 rollers inreturn strand.

Power calculation

PT= P1+P2+P3+P4+P5+P6+P7+P8+P9

P1 = Friction in support rollers.P1= 0.1[ (L1+W1+(Z x r) ]

Carrying strand:L1 = Load on belt for belt carried by rollers =60 x 150 = 9000 kgW1 = Weight of belt times length of belt = 9.42 x 150 = 1413 kgZ = Number of idlers = 81r = weight of rotating part of roller = 30 kgInserted values give P1 = 1284 NReturn strand:L1 = 0W1 = 1413 kgZ = (150-6)/8+2 = 20r = 30 kg

Inserted values give P1 = 201 N

P1tot = 1284 + 201 = 1485 N

P2 = Rubber lagged rollers

P2 = 0.16 (L2 + W2)

L2 = Load on belt times the rolling length= 60x150 = 9000 kg

W2 = Weight of belt times the rolling length= 9.42x150x2 = 2826 kg

Inserted values gives P2 = 1890 N

P3 = Friction from slide supports = 0

P4 = Pull to accelerate the load

P4 = k2 x qg

k2 = 9 ( belt speed >60m/min )

qg = 60 kg/m

Inserted values give P4 = 540 N

P5 = Friction from scrapers during discharge P5 = qg (15 Bm + 2 )

qg = 60 kg/mBm = Belt width = 1.0 m

Inserted values give P5 = 1020 N

P6 = Friction from terminal bearings

P6 = 0.01 x 20 x B x t

B = Belt width = 1000 mmt = Belt thickness = 1.2 mm

Inserted values give P6 = 240 NP7 = Friction from safety devicesP7 = 500 x Bm

Inserted values give P7 = 500 NP8 = Pull from incline = 0P9 = Friction from side skirts

P9 = k3 x L4

k3 = 0.5

L4 = 150x60 = 9000

Inserted values give P9 = 4500 N

Add all contributions and the sum will be:PT = 10175 N

19

Drive shaft power

E = (1.2 x P x V)/60000

Inserted values give E = 26.5 kW.

Efficiency for transmission must be added toarrive at required motor power.

Belt tension

According to the formula on page 8 the mini-mum belt tension should be Qp = 3.3 x PT

This gives Qp = 3.3 x 10175 = 33600 N

Note that Qp is larger than required minimumpre-tention (Q).

Use 33600 N as pre-tension in this case.Determine tensile stress:

σ = (Qp/2 + PT)/(1000x1.2) = 33600/2 +10175/(1000x1.2) = 22.5 MPa

Determination of drum diameter

See figure 8: (v x s)/ L = (130x 2)/150 = 1.73This gives K = 800Minimum drum diameter 800 x 1.2 = 960, say1000 mm.

Belt sag in carrying strand

f = [ 5 x 103( q + qg ) x a2 ] / ( 4 x S ) see page 10.S = Qp /2 = 16800 N, a = 1.85 mq = 9.42 kg/mqg = 60 kg/m gives f = 18 mm

Note that the belt sag could be reduced byhigher pre-tension or shorter idler spacing.

Sandvik 1200SA

Belt Width mm (inches)

200 300 400 500 600 800 1000 1200 1400 1500 1560 2080 3000* 4500**8 12 16 20 24 32 40 48 55 59 61 82 118 177

Thickness mm (inches)

0.4●

(0.016)

0.6● ● ● ● ●

(0.024)

0.8● ● ● ● ● ●

(0.032)

1.0● ● ● ● ● ● ● ● ● ●

(0.040)

1.2● ● ● ● ● ● ● ●

(0.048)

1.6● ● ● ● ● ● ● ● ●

(0.063)

2.0● ● ● ● ●

(0.079)

Sandvik 1150SM/1650SM/1500SM

Belt Width mm (inches)

800 1000 1200 1400 1500 1560 3000* 4500**32 40 48 55 59 61 118 177

Thickness mm (inches)

0.8●

(0.032)

1.0● ● ● ● ● ● ●

(0.040)

1.2● ● ● ● ● ● ●

(0.048)

1.6● ● ● ● ● ● ●

(0.063)

1.8● ● ● ● ● ● ●

(0.071)

2.0● ● ● ● ● ● ●

(0.079)

2.3● ● ● ● ●

(0.091)

2.7● ● ● ● ●

(0.106)

3.0● ● ● ● ●

(0.118)

Sandvik 1300C/1320C (no stock standard)

Belt Width mm (inches)

200 300 400 500 600 800 1000 1200 1250 2380* 3560** 4500***8 12 16 20 24 32 40 48 49 94 118 177

Thickness mm (inches)

0.6● ● ●

(0.024)

0.8● ● ● ● ●

(0.032)

1.0● ● ● ● ● ● ● ● ● ●

(0.040)

1.2● ● ● ● ● ● ● ●

(0.048)

1.4● ● ● ● ● ● ● ●

(0.055)

Sandvik 1000SA

Belt Width mm (inches)

600 800 1000 1200 1400 1500 1560 3000* 4500**24 32 40 48 55 59 61 118 177

Thickness mm (inches)

1.0● ● ● ● ● ● ● ● ●

(0.040)

1.2● ● ● ●

(0.048)

Sandvik 1050SM

Sandvik 1100C: Belt widths 1250-1500 mm (49-59 inches),thickness 1.2 mm (0.048 inch).

Sandvik 1500SM: Belt widths 1200-1560 mm (47-61inches), thicknesses of 1.2, 1.8, 2.7 and 3.0 mm (0.048, 0.071,0.106 and 0.118 inch).

Stock standard dimensions indicated within thick lines.* max. width with one longitudinal weld.** max. width with two longitudinal welds.*** max. width with three longitudinal welds.

Standard dimension rangefor Sandvik steel belts

Belt Width mm (inches)

600 800 1000 1200 1400 1500 1560 3000* 4500**24 32 40 48 55 59 61 118 177

Thickness mm (inches)

0.8● ● ●

(0.032)

1.0● ● ● ● ● ● ● ● ●

(0.040)

1.2● ● ● ● ● ● ● ●

(0.048)

Belt width mm (inches)

100 200 300 400 500 600 800 1000

(4) (8) (12) (16) (20) (24) (32) (40)

Thickness mm (inches)

0.2 (0.008) ● ● ● ● ● ●

0.3 (0.012) ● ● ● ● ● ● ●

0.4 (0.016) ● ● ● ● ● ● ● ●

0.6 (0.024) ● ●

Sandvik 1700SA

Sandvik Mater ia ls Technology Process Systems, SE-811 81 Sandviken, Sweden, Phone +46 26 56 20

www.smt . sandv ik .com/sps PS-

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