T

o optimize the performance of anyspeed reducer, the gears must be ori-ented to each other so that they runsmoothly without binding or interfer-ence. They must also maintain the cor-rect amount of tooth clearance (back-lash) at all operating conditions. For spurand helical gears, the best orientationnormally requires only that the centerdistance and shaft alignment be correct— no further adjustment is needed.

This is seldom the case with bevelgears, where mounting arrangements al-low for wide variations in position. Alltypes of bevel gears — straight, Zerol, spi-ral, and hypoid — have an optimum posi-tion for best performance. The manufac-turer determines this optimum positionby running tests of individual gearsets.But, the gearbox designer and assembly

technician share the responsibility for in-corporating this optimum positioning inthe gearbox. The designer must providefor ease of measurement, shimming, andassembly. And the technician must prop-erly assemble the gears in the gearbox.

Key assembly parametersSeveral parameters contribute to

proper assembly so that the gearbox op-erates smoothly and efficiently. The mostimportant are:

• Mounting distance.• Matched teeth.• Backlash.Bevel gears of AGMA quality 8 or bet-

ter are normally manufactured andtested in sets. Then, the manufacturemarks the preferred values for each of

the three parameters on individual gears.Frequently, the manufacturer also

marks a gearset number on each of thetwo gears in a set. Gears manufactured insets should

only be assembled with theirmates. Similarly, if one gear in a set fails,the entire set should be replaced.

Mounting distance. The distance froma locating surface on the back of one gearto the centerline of a mating gear iscalled the mounting distance, Figure 1.This is the most important parameter forensuring proper operation. The manufac-turer establishes the optimum value forthis distance by running the gearset andadjusting its position to obtain a toothcontact pattern that is consistent withsmooth running and optimum load distri-bution between mating gear teeth. Be-cause of dimensional variations betweenparts, each gear in a set has a uniquevalue for the mounting distance and, inmost cases, the manufacturer perma-nently marks this value on each gear, Fig-ure 2.

It is possible to manufacture all gearsto the nominal mounting distance speci-fied on the drawing. But, the additionalcost to do so is usually not warranted.

In cases where mounting distanceshave not been marked on the gears, as-sembly technicians must resort to acostly trial and error method of markingteeth with gear compound and adjusting

How to install bevel gears for peak performance

You can’t put bevel gears together in the same way as spur andhelical types and get them to work well. They need to be assembled in

a specific way to ensure smooth running and optimum loaddistribution between gears.

ROBERT F. WASILEWSKI, Arrow Gear Co.

POWER TRANSMISSION DESIGN

■ MARCH 1994 51

PRODUCT FOCUS:

GEARING

Robert F. Wasilewski is the design engineer-ing manager for Arrow Gear Co., DownersGrove, Ill.

Figure 1 — Mounting distances used for assembling bevel gears.

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Mounting distance for pinion

Pinion centerline

Mounting distance for gear

Gear centerline

Crossing point

Back angles

#11444 FIG 1

10C-20M-40Y

20C-50M-60Y

10C-30M-50Y

30C-20M-50Y

10C-10M-50Y

30C-20M-10K

50C-15M-20K

20K

10C-20M-70Y

15C-70M-50Y

the gears to obtain a suitable tooth con-tact pattern. The lack of marked valuesmay also indicate that the gearset wasnot manufactured for high capacity orconsistency.

Matched teeth. After optimum toothcontact is obtained in the running tests,the manufacturer marks mating teeth inengagement for identification. Thesemarks usually consist of x’s or dots, Fig-ure 2, on two adjacent teeth of one gearand the mating tooth of the other gear.When assembling the gearset, positionthe single marked tooth in the space be-tween the two adjacent marked teeth.

Backlash. The third most importantparameter for a bevel gearset is the spacebetween mating gear teeth, called back-lash. Unless otherwise specified, back-lash is measured normal (perpendicular)to the tooth surface, Figure 3, and not inthe plane of rotation. In most cases, themanufacturer marks the normal back-lash value on the gear, Figure 2.

Measurements in the plane of rotation,called transverse backlash, are as muchas 40% larger, and should not be used forassembly purposes.

Where mounting distances have beenmarked on one or both gears, adjust thesedistances first. Then, use the normalbacklash value to verify proper assembly.The backlash value should not be mea-sured until after assembling at least one

of the two gears, preferably the pinion,at its marked mounting distance. Thereason is simple. Because bevel gears areconical in shape, they can be assembledin an almost infinite number of positions(most of which cause poor performance)and still obtain the required backlash.

Frequently, a casual user assemblesbevel gears to obtain a specific amount ofbacklash without regard to the mountingdistance. This is especially true for lowquality or lightly-loaded bevel gears. Al-though this method occasionally works, itis a risky approach where gears areloaded to maximum capacity. It usuallycauses shorter life and poor perfor-mance. Only at the proper mounting dis-tance will a gearset run correctly and stillhave the right amount of backlash.

Gear assemblyIn most bevel gearsets, particularly

those with ratios above 2:1, the pinion po-sition (mounting distance) effects toothcontact (the most important parameterfor good performance) to a larger extentthan the gear position. Conversely, thegear position has a larger effect on back-lash. For this reason, the manufacturermay have marked the mounting distanceonly on the pinion. In such cases, be sureto accurately position the pinion accord-ing to its marked mounting distance be-

fore adjusting backlash. This can be ac-complished either by gaging or by directmeasurement. Gages are used mostly forlarge gear production runs, and are basedon the direct measurement method. Forthis reason, only the measurementmethod is described here.

Note that when mating gears are ad-justed to their optimum position, theirback angles, Figure 1, will probably notbe flush with each other. Do not attemptto position bevel gears by making theback angles flush.

Measurement method. A typical gear-box, Figure 4, contains both an overhungand a straddle-mounted gear. The proce-dure for assembling the bevel gears in thisgearbox by the measurement method issimple:

• On the housing, measure the distancefrom locating surface to bore centerline inboth horizontal and vertical directions(HMD and VMD).

• Record the gear and pinion mountingdistances (MDG and MDP).

• Measure the gear and pinion thick-nesses (WG and WP).

• Assemble the gears into their sub-assemblies.

• Measure the length that controls gearposition in each subassembly (MAG andMAP).

• Calculate the distance from shimmounting surface to crossing point for eachsubassembly (MDV and MDH).

• Calculate the required shim thick-nesses and assemble the gearbox withshims in place.

To aid the assembly technician, markthe housing measurements on the housingafter it is machined and inspected.

Measurements involving the pinion andgear are taken on the subassemblies, whichconsist of gear, shaft, bearing, and gearmounting components. This minimizes anyvariations due to fits between these compo-nents and greatly reduces the number ofmeasurements required. In this example,only the gear thickness is measured beforeputting together the subassembly. Sub-tracting this distance from the mountingdistance lets the technician measure fromthe front of the assembled gear.

POWER TRANSMISSION DESIGN ■ MARCH 199452

PRODUCT FOCUS: GEARING

#11444 FIG 2

X X X

Figure 2 — Typicalmarkings on bevelgears include (left

to right) the gearsetnumber, mounting

distance (MD) ininches for each

gear, x’s on matingteeth, and the

backlash dimension(B/L) in inches.

Verifying properassembly. Use themeasurementmethod to assemble both gears, makingsure that the matched teeth are properlyengaged. Only after positioning both

gears at their mounting distances shouldyou measure backlash to verify proper as-sembly. In cases where only the pinion is

marked with a mounting distance, posi-tion the pinion at its mounting distancefirst, then adjust the gear position for therequired backlash.

To obtain further verification, you canperform an optional contact patterncheck. This involves painting the toothsurfaces on one gear with a thin markingcompound and rotating the assembledgears under light load. The compoundtransfers from one gear to the other andshows how the teeth contact. Numerouspublications contain charts with typicalcontact patterns (ANSI/AGMA 2008-B90and ANSI/AGMA 2005-B88). The patternsseldom look exactly like the published ex-amples and frequently require some in-terpretation.

Technicians familiar with contact pat-tern checking may believe that this tech-nique is the preferred method of assem-bly. However, the manufacturer hasalready performed this check as part ofthe testing (to determine mounting di-mensions) and interpreted the contact

patterns so you don’t have to.■

Figure 4 — Measurementsrequired for mounting bothoverhung and straddle-mountedbevel gears in a typicalgearbox.

POWER TRANSMISSION DESIGN ■ MARCH 1994 53

Backlash measurement

Normal backlash

Top view of teeth

Tooth outer end

Pitch circle

#11444 FIG 3

Figure 3 — Normalbacklash in a set of

spiral bevel gears ismeasured perpen-dicular to the toothsurface at the outer

end (heel) of thetooth, using a

dial gage.

#11444 FIG 4

Straddle mounted gear subassembly Assembled gearbox

Overhung mounted pinion subassembly

Shim thickness = MDV – VMD

Shim thickness = MDH – HMD

Housing

HMD

VMD

MDV

MAP

WP

MAG

MDG

WG

MDP

MDH

MDH =

MDV = MD

G – W

G + MA

G

MDP – W

P + MA

P