Question 1

How do I calculate the speed ratio of two pulleys?

Accepted Answer

The speed ratio is the driven diameter divided by the drive diameter: i = D2 / D1. For example a 100 mm drive pulley turning a 200 mm driven pulley gives i = 200 / 100 = 2, written 2 : 1. The same ratio applies to the teeth count on timing pulleys.

Question 2

How fast does the driven pulley turn?

Accepted Answer

The driven speed is N2 = N1 · D1 / D2, where N1 is the drive speed. A larger driven pulley turns slower. With N1 = 1450 rpm, D1 = 100 mm and D2 = 200 mm, N2 = 1450 × 100 / 200 = 725 rpm.

Question 3

How do I work out the belt (surface) speed?

Accepted Answer

Belt speed is the rim speed of the drive pulley: v = π · D1 · N1 / 60000, giving metres per second when D1 is in millimetres and N1 in rpm. For example π × 100 × 1450 / 60000 ≈ 7.59 m/s.

Question 4

What is the open-belt length formula?

Accepted Answer

For an open (uncrossed) belt with centre distance C: L = 2C + (π/2)(D1 + D2) + (D2 − D1)² / (4C). With C = 400 mm, D1 = 100 mm and D2 = 200 mm, L = 800 + 471.24 + 6.25 ≈ 1277.5 mm. This is the theoretical pitch length before rounding to a stock belt.

Question 5

Does this account for belt slip and tension?

Accepted Answer

No. These are idealised results for an open belt assuming no slip, a rigid massless belt and pulleys running at their pitch diameter. Real V-belts slip slightly, must be rounded to a standard catalogue length, and need correct tension and a wrap-angle check. Use the figures for sizing, not as exact final values.

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