Properly Align Variable-, Fixed-Pitch Sheaves
Jane Alexander | January 13, 2017
Variable-pitch sheaves are frequently used in air handlers. According to a blog post by Stan Riddle of VibrAlign (Richmond, VA, vibralign.com), they allow design engineers to increase or decrease the speed of the driven machine and, thus, provide:
• changes in motor amp draw to maximize efficiency
• increased or decreased static pressure and air flow.
Normally, a design engineer will specify the use of a variable-pitch sheave on the driver and a fixed-pitch sheave on the driven machine.
Performed on a single-belt machine, proper sheave alignment is simple, if a good sheave-alignment tool is used. When multiple belts are used, as they often are, proper sheave alignment can become more complex. A variable-pitch sheave can be adjusted to increase/decrease the sheave diameter. However, doing so also changes the sheave width, depending on the adjustment.
In his post, Riddle referred to a customer who was attempting to perform a sheave alignment on an air handler. The unit’s motor had a variable-pitch sheave, but the fan sheave was fixed. The customer stated that he could align one belt, but not the other.
As Riddle described it, the customer was struggling because the width of the fixed-diameter sheave was 1 5/8 in., but the width of the variable-pitch sheave was 2 3/8 in. Consequently, only one set of grooves could be aligned, meaning the other was out of alignment.
Riddle wrote that the solution to the customer’s problem was simply to split the difference between the width of the two sheave diameters, as shown in the following equation:
2 3/8 in. – 1 5/8 in. = 3/4 in. ÷ 2 = 3/8 in. offset on each groove
Riddle also noted that it’s important to keep in mind this approach will probably not align the components sufficiently to eliminate sheave and belt wear. In fact, such wear can’t be eliminated. Still, when it comes to aligning multiple V-belts on an equipment system, splitting the difference between the diameter width of a variable-pitch sheave and that of a fixed-pitch sheave to which it is aligned will make the belts wear evenly.
Variable-pitch sheaves are normally used to balance a system and achieve proper static pressure and speed. When that’s determined, according to Riddle, 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.
Stan Riddle, a technical trainer for VibraAlign, has spent more than 36 years aligning industrial machinery. For more information from him and other technical experts with the company, visit vibralign.com.