Pulley, Belt, and Sheave Sizing for Car Wash Pumps and Motors

Connecting pumps to motors is integral to setting up your pump room. For many of them, you’ll want to connect them via drive belts. But most pumps simply come with an unadorned shaft.

This is where pulleys and sheaves come into play. These allow the shafts to transfer power via a belt. To get the best performance, you need to make sure that you get the correct pulleys for each application. If you’re off, it can cause issues with performance. If you transfer RPM where it is slower than recommended, this can lead to lackluster performance by the pump. If the transferred RPM causes the pump to turn too fast, it can cause mechanical failures including valve wear and elastomer failure.

So knowing this, how do you make sure that you have the right pulley sizing for your system?

AMEC Pulley

The Math of Pulley and Sheave Sizing

It all starts by understanding the RPM output of your drive component – an electric motor, a gas-powered engine, or other driver – as well as knowing the RPM needed by the pump you want to drive.

After that, you need to solve the equation knowing what you have. In the end, the pulley diameter for your pump will have to equal the driver pulley diameter multiplied by the motor RPM divided by the pump RPM.

Let’s consider that you have a 1725 RPM motor, and a pump that needs to run at 1100 RPM. This makes the last part of the equation 1.568. If you have a 4-inch driver pulley, you’ll multiply that by the last part of the equation (1.568) to get the pulley sizing needed for the pump pulley – in this case, 6.27”. This may be difficult to source, in which case you may want to change sizing of the motor pulley. By guessing and testing, you could try sizing up the driver pulley to 4.5”, a fairly common measurement. Solving the equation, this comes out to a 7.056” pulley – more than close enough to the fairly standard 7” pulley size!

Tapered Bushing or Fixed Shaft Bore?

You may need to alter the shaft of your motor or pump to fit the correct pulleys you need. Finding the pulley bored to the size of the shaft versus utilizing a tapered bushing is something you’ll need to decided. Both of these have advantages and disadvantages.

The biggest advantage of attaching a tapered bushing to the shafts of your pump or motor is that it allows for swapping out pulleys without having to worry about new bores – the pulley will just slide down the taper until it is secure. The tapered bushing also hugs the shaft better thanks to the materials creating friction, meaning less likely slippage. Over time, a fixed bore can wear out of tolerance and wiggle, where with the tapered bushing that bore can just continue to slide to the part of the bushing where it will hold.

A fixed shaft bore, on the other hand, has the advantage of being a single piece. It’s lighter, and easier to work with as there is no loosening or tightening, adjustments, or issues over time. They potentially will have a truer center on the shaft. They are also easier and less fussy to implement.

Choosing between the two is really more a matter of your situation. Need versatility? The tapered bushing is a great choice. Want simplicity? The fixed shaft bore pulley will be the better choice.

Continental BX Belt

Belt Type

The type of belt you use with your pulleys and sheaves will make a difference. The most common belt used in the pulleys and sheaves for driving pumps is a V-belt. These are split into two designations – A belts and B belts.

A belts are 1/2 inch in width across the widest part, and have a thickness of 5/16 inch. B belts, on the other hand are 21/32 inch at the widest and have a thickness of 13/32 inch. Thus, A belts will sit lower in the pulley groove than a B belt. This, in turn, will affect the length of belt you get.

You may also see AX and BX belts. These aren’t different in terms of measurement from standard A/B belts. Instead, the X denotes that these belts are cogged, which are good for use with small-diameter pulleys, provide great heat dissipation from friction, and help to reduce slippage.

Belt Sizing

Getting the right belt to go with those pulleys is important as well, for non-slipping power transfer. A loose belt will cause slippage and poor power transfer, while a tight belt could cause the motor or pump to lock up, or may be impossible to even seat if too small!

To get the correct belt size, you’ll need to use yet another mathematical equation, that takes into account the radius of the pulleys as well as the distance between the two pulleys, as well as the fact that the belt is a loop.

Belt Size = (Pump Pulley Outside Diameter x 1.57) + (Motor Pulley Outside Diameter x 1.57) + (Distance between Center of the Pulleys x 2)

So let’s say the motor and pump mentioned above, with the 4.5” and 7” pulley, are spaced so that the center of the pulleys are 30 inches apart. Your equation would be:

(4.5 x 1.57) + (7 x 1.57) + (2 x 30) = 78.055

You would want to get a 78” belt. This equation can be used to determine any belt length needed!

Getting the right pulley and sheave set-up to work with your car wash pump and motor is incredibly important! It’s the difference between getting peak performance and longevity, or having lackluster performance or having to deal with maintenance issues. So make sure to get it right – reach out to our team here at Kleen-Rite if you have questions about finding the right pulley, sheave, or belt for your car wash application!