Variable pitch sheaves are used frequently in air handlers. They allow the design engineer to increase or decrease the speed of the driven machine. In doing so, they allow for:
- Changes in amp draw of the motor, to maximize efficiency,
- Increase or decrease static pressure and air flow.
Normally, the design engineer will specify the use of a variable pitch sheave on the driver, and a fixed pitch sheave on the driven machine.
When used with a single belt design, proper sheave alignment is simple, if a good sheave alignment tool is used. However, when multiple belts are used, as they often are, proper sheave alignment can become more complex. A variable pitch sheave can be adjusted to make the diameter of the sheave bigger or smaller. However, this also makes the width of the sheave wider or narrower, depending on the adjustment.
We recently fielded a customer call who was attempting to perform a sheave alignment on an air handler, using a VibrAlign Belt Hog. The motor had a variable pitch sheave, but the fan sheave was fixed. He stated that he could align one belt, but not the other.
Here’s why he was struggling.
The width of the fixed diameter sheave is 1-5/8”, but the width of the variable pitch sheave is 2-3/8” inches. So, only one set of grooves could be aligned, meaning the other was out of alignment.
The answer? Split the difference.
2 3/8” – 1 5/8” = ¾” ÷ 2 = 3/8” offset on each groove.
It is important to note that this will probably not align the sheaves sufficiently to eliminate wear on the sheaves and belts. Nor can it be eliminated. It will however make the belts wear evenly.
Variable pitch sheaves are normally used to balance a system out, and achieve proper static pressure and speed. Once that is determined, the variable pitch sheave should be replaced with a fixed pitch sheave of the proper diameter to match the desired speed and pressure. Once both sheaves are fixed pitch, proper alignment can be achieved.
Do you have belt alignment questions? Call VibrAlign at (800)394-3279, or contact us at VibrAlign.com.
I have a question i hope you can help with. I hap a vp60 varible pitch, 2 groove sheave. We ordered a vp60 fixed pulley thinking it should match up. The belt sits all the ay on the outside of the pulley so it is spinning to fast and causing the motor to overload. My question is when replacing a variable pitch sheave, how do you know what size fixed pulley to get. I thought i remember something about the size stamped on the sheave and the number of turns its spun out from the center. Besides that im quite lost. thank you
Good question, Tony. Variable pitch sheaves are designed for temprorary use, so that speeds can be adjusted. Once the correct speed is chosen, the variable pitch sheave should be replaced with a fixed sheave. The easiest way to measure for the fixed pitch size is to measure the diameter of the belt wrapping around the sheave. As an example, a variable pitch sheave may be adjustable from 6.2 to 9.5 inches diameter. Once the proper speed is achieved, measure across the sheave. Measure from the center of the belt on top of the sheave, to the center of the belt on the bottom of the sheave. This is the running diamter of the belt. Usually measured in .1 inch dimensions. Hope this helps!
Sir I have been the ac building maintenance business since 1979 and just have one question on adjustable shaves I see them every wharere and the belts and sheaves are always wearing out quickly. Should all the adjustable sheaves be replaced with set sheaves after air balanceing is complate? This has botherd me ever since I saw one of these type of sheaves, the belt never rides at the right point in the sheave.
You are correct. Adjustable sheaves should be for temporary use, to air balance a system. After that, they should be replaced. But often they are left in place. So instead of replacing the adjustable, you replace a load of belts, and two sheaves!