Design notes

1.Adjustment for the centre distance must be provided in both directions from the nominal value to set the initial tension. Provision for increasing the centre distance must be made to take up for belt stretch during use. If fixed centres are required, idler pulleys should be used to adjust the belt.

2.Ideally, the nominal range of centre distance should be D2 < C < 3*(D1+D2)3.Maximum centre distance should be less than 20 times the smaller pulley

pitch diameter.4.The wrap angle on the smaller pulley should be greater than 120°.5.Belt speed should be <30m/s (6,000 ft/min).6.Consider an alternative type of drive, such as a gear type or chain, if the

belt speed is less than 5 m/s (1,000 ft/min).7.Avoid elevated temperatures around belts.8.Avoid contaminants such as oil, grease or grit on the belts.9.Ensure that any shafts with pulleys are parallel, and the pulleys are in

alignment.If an idler pulley or roller is used, it should be installed on the slack side of the belt. It should also be closer to the smaller pulley to maximise the wrap angle of the belt. A typical arrangement is shown at left. The idler pulley position should be adjustable to 1) set the initial tension, and 2) to adjust the tension as the belt ages and stretches during use.

It is possible for lower power applications to make a primitive clutch using a belt and an idler. Referring to the diagram just above, the centre distance is deliberately made too large so that the belt slips on the pulleys and no power is transferred. The idler roller is then moved inwards to increase the belt tension until the belt fully engages. While possible, this is a very simple and crude arrangement, but can be effective in some applications. It will require trial and error to align for desired operation, will increase the wear on the belt

shortening its life and require more space for the idler movement and operating mechanism.

Correct belt pulley selection.Important factor of reliability and equal operation of V-belts is exact fulfilling of conditions specified forbelt pulleys:• belts should work at grooved pulleys of dimensions fitted to belt section, as only side (working) wallsof belt were in contact with walls of pulley grooves;• pulley grooves should be smooth, without deformations, snagging and contaminations, particularlygrease and oils. Surface coincides of groove pulleys should not be painted.• all grooves edges of belt pulleys should be rounded of radius r>1mm;• minimal recommended diameter of pulley for given belt section should be observed unless compactnessof gear is going to be reached at all costs, even at the expense of reduced

efficiency and loweringof belt lifetime;• all wheel belts should be balanced statically (degree of quality Q in accordance to VDI 2060); additionallyfor wheel belts which peripheral speed exceed 30 m/s, or if a ratio of diameter to width of pulleyrim dp/s<4 while v>20 m/s, should be balanced dynamically (degree of quality Q 6.3);• with respect to belt lifetime is recommended to use pulleys of big diameters, for which belt speed shouldbe between 25-30 m/s;• grooved pulleys should be made according to recommendations included in chapter 4.

V-belt drives are essentially short centre drives. If in drive design the centre distance   C is not specified, then it should be set at around   2D1 √ (R+1)   but preferably not less than   D2 . Since the diameters and belt length

are discrete variables so also is the theoretical centre distance, though in the absence of idlers the nominally fixed centre distance must be capable of slight variation by motor slide rails for example, to allow for belt installation and subsequent take-up (initial tightening) before rotation commences. This capability also allows for manufacturing tolerances on belt length, L. From the geometry :-

( 3)     ( Fmax - ρv2 ) / ( Fmin - ρv2 )   ≤   e ( f θ)min

                                    where   f ≡ μ∗cosecβ     and   ( f θ)min   =   min (   (fθ)1 , (fθ)2 )

Every cross-section of the belt is subjected alternately to the strand tensions   Fmax and   Fmin and therefore undergoes fatigue. These tensions at least must be detemined before fatigue can be addressed. If the power transmitted by the drive   P is shared equally between   z belts in parallel then since the torque per belt always equals   ( Fmax - Fmin) ∗ radius :-

( 4a)       P   =   z ( Fmax - Fmin) v       ie.       Fmax - Fmin   =   P / z v

This relation alone is insufficient for determining   Fmax, Fmin individually for a given power- per- belt (P/z) and velocity (v). If however we focus on the maximum power transmissible without gross slip, then   equation ( 3) applies. Eliminating   Fmin between ( 3) and ( 4a), the tight side tension   Fmax per belt for a given slip- limited power P is :-

( 4b)       Fmax   =   P / z kθ v   + ρ v2     where the drive property     kθ ≡ 1 - e - ( f

θ)min     by definition.

Properties found by curve fitting to belt capacities cited in the Code, are given in Table 1.

    STANDARD PITCH LENGTHS ( mm )

Y Z A B C D SPZ SPA SPB SPC   

200405 630 930 1560 2740 630 800

1250

2000

224 475 700100

01760 3130 710 900

1400

2240

250 530 790110

01950 3330 800

1000

1600

2500

280 625 890121

02190 3730 900

1120

1800

2800

315 700 990137

02340 4080

1000

1250

2000

3150

355 780110

0156

02490 4620

1120

1400

2240

3550

400 920125

0169

02720 5400

1250

1600

2500

4000

450108

0143

0176

02880 6100

1400

1800

2800

4500

500155

0195

03080 6840

1600

2000

3150

5000

1750

2180

3310 7620180

0224

0355

05600

1940

2300

3520 8410200

0250

0400

06300

2050

2500

4060 9140224

0280

0450

07100

2200

2700

46001070

0250

0315

0500

08000

2300

2870

53801220

0280

0355

0560

09000

2480

3200

61001370

0315

0400

0630

01000

0257

0360

06860

15200

3550

4500

7100

11200

2700

4060

7600 8000

12500

2910

4430

9100

3080

4820

10700

3290

5370

3540

6070

The design power input to the selection process must incorporate a duty factor to allow for shock, high starting torques and other expected non-uniformities :-

( iii)       design power   =   actual nominal power ∗ duty factor (from the table below eg.)

Note that a duty factor of 1.5 implies a life reduction of approximately   1.512 = 130.

  RECOMMENDED PULLEY PITCH DIAMETERS ( mm )Y Z A B C D SPZ SPA SPB SPC

20 50 75 125 200 355 67 100 160 224  22.4 53 80 132 212 375 71 106 170 236

25 56 85 140 224 400 75 112 180 25028 60 90 150 236 425 80 118 190 265

31.5 63 95 160 250 450 85 125 200 28035.5 67 100 170 265 475 90 132 212 315

40 71 106 180 280 500 95 140 224 35545 75 112 190 315 530 100 150 236 40050 80 118 200 355 560 112 160 250 45056 85 125 212 400 630 125 180 280 50063 90 132 224 450 710 140 200 315 56071 95 140 236 500 800 150 250 355 63080 100 150 250 560 900 160 315 400 80090 112 160 280 630 1000 180 400 500 1000

100 125 180 315 800 1250 200 500 630 1250112 140 200 355 1000 1600 250 630 800125 150 250 400 1250 2000 315 800 1000

160 315 500 400 1000 1250180 400 630 500200 500 800 630250 630 1000 800315 800 1250400 1000500630800