Pulley dimensions include belt length, speed ratio, and belt pitch diameter. Belt length can be measured with a string and measuring tape. Speed ratio can be calculated using RPM or pulley circumferences. Pitch diameter is difficult to measure, but speed ratio is important for determining mechanical advantage. Pitch diameter can be found in manuals or online calculators.
There are many pulley dimensions to achieve from a pulley system, including belt length, system speed ratio, and belt pitch diameter. The speed ratio is calculated using the revolutions per minute (RPM) or the circumferences of the engine and driven pulleys. While the pitch diameter of the belt is often not needed, this dimension can be difficult to determine.
The first dimension to measure is the length of a belt between two pulleys. An easy way to do this is to take a piece of string, wrap it around the two pulleys and mark where the end of the string touches another part of the string. Take a measuring tape or yardstick and measure from the end of the string to the marked part of the string.
The next of the pulley dimensions you can measure is the speed ratio; to obtain this measurement, both the RPM and the circumferences of the driving pulley and the driven pulley can be used. For a pulley system such as those found in an automobile engine, RPM can be found using a particular engine’s printed or online manuals or the use of a tachometer. You probably already know the drive pulley RPM, but not the driven one. If you know the RPMs of both pulleys, you can divide the drive pulley RPM by the driven pulley RPM. If not, the circumferences of the drive pulley and driven pulley must be used to determine the speed ratio.
Obtain the circumferences of both the drive pulley and the driven pulley by measuring the radii of both pulleys. To determine the radius, measure across the width from one edge of the pulley to the center. Multiply the radius by 2 π (pi) to get the circumference of the pulley. Perform this operation for both pulleys to obtain two circumference values. Then, divide the circumference of the driven pulley by the circumference of the drive pulley to get the speed ratio of the system.
Speed ratio is one of the essential pulley dimensions to know for any pulley system found in machinery such as automobile engines or industrial conveyor belts. It is used to determine how much work is being done and how much power is being exerted in a system, known as mechanical advantage. As an example of this concept, bicycles have multiple gear settings, or speed ratios, to increase or decrease the workload a rider uses to pedal. In automobile engines, speed ratio is used to increase the output torque or output speed.
One pulley size that you may need, but not necessarily be able to obtain, is the pitch diameter of a belt. This is the diameter of the threads that are inside the belt. It is possible, but impractical, to measure this. Instead, you can consult a manual on a particular piece of machinery or use an online calculator with the system’s measured pulley size.
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