Sheaves – What Are They, Calculating Sheave Pitch, & More

sheaves

A common sheave.

There is a lot of confusion in our building engineering and facility maintenance worlds when it comes to sheaves. Today, we are going to:

  • Define sheaves and what they are.
  • Cover fixed and adjustable sheaves.
  • How to figure out motor sheave pitch diameter.
  • Common sheave materials.
  • Learn how you can change fan speed with a sheave.

Sheaves can be confusing because some terms, like pulleys, are interchanged kind of depending on where you work or what you do. You might get told to go order a sheave but your supervisor really means a pulley. Or you may get asked if you adjusted the sheaves. Well if it’s a fixed diameter sheave you can’t adjust it like you could a variable one.

For our purposes, sheaves are going to drive belts or v-belts. If it uses rope or cable, we’ll call that a pulley for now. That may or may not be correct technically but for us, in our industries, we generally call belt driven pulleys, sheaves. Okay?



What Are Sheaves?

adjustable sheaves

Sheave with the belt removed.

Sheaves are mainly used to guide or hold belts or v-belts, rope, cable. A sheave is basically a wheel with a channel or groove sandwiched between the two edges of the wheel.

And for all of you that are wondering how to say the word sheave, it is pronounced shiv. Like the prison knife I guess. You don’t say it sheave like leave but shiv like Liv, as in Tyler. Yum. Anyhoo.

According to wikipedia, when one or more sheaves is hung between two supports and equipped with a belt, cable, or rope, that is then called a pulley. Clear now yeah?

  • Sheaves in essence, transmit power.

Maybe? They give us the ability to like transfer or re-locate where, or how, energy is expended and then to where the work is done. Remember that power is the rate at which work is done.

So, our building has a fan up on the roof. At one end of the fan shaft, way more on this below from another article, a sheave is mounted. It might be fixed, it could be adjustable or variable, heck it could have multiple grooves or channels.

Then say right below it would be a motor drive by electricity, or steam even, with it’s own motor sheave installed. Connecting the two sheaves? A belt, rope, or cable son.

Sheaves take the electrical energy that turns the motor, transmit that power through a belt, onto the other sheave, which turns the shaft, which turns the fan, which moves the air.

  • We have now converted that initial electrical energy into mechanical energy that moves the air.

Sheaves can drive supply and exhaust fans, big air handling units, pumps, etc. They are versatile and have the ability to drive many things. Usually there is a driven sheave and then a driving one. In our example, the motor would be the driving sheave and the fan sheave would be the one being driven.

Fixed Sheaves

v-belt sheave

Sheave with the v-belt in place.

A fixed sheave is pretty self explanatory. You can’t adjust them, lol. You replace them based on size and technical data.

When replacing a fixed sheave you should look at, at least:

  • Outside diameter
  • Bore hole diameter
  • And sheave width.

These are all important factors when replacing sheaves. If you get a sheave with the incorrect bore hole diameter, it won’t fit. If you get one with too narrow of a groove, it won’t fit the belt that was designed to run it.

Outside diameter is especially critical for a fan or piece of equipment that requires a specific size sheave. If you change the sheave size, you are basically modifying the piece of equipment that turns as a result.

Let’s pretend our two sheaves, the driving and the driven, each rotate at a 1:1 ratio. Meaning every time the motor sheave turns once, so does the fan sheave.

Now, what if we increased the size of the sheave on the fan shaft? Let’s say we accidentally doubled the size of the outside diameter. Now, every time the motor sheave turns once, we will have only rotated the fan sheave 1/2 a turn. This will decrease our desired air flow. You can find more on how to change fan speed using an adjustable sheave below.



Adjustable Sheaves

what is an adjustable sheave

An adjustable sheave, the two pieces are screwed together.

Adjustable or variable pitch sheaves can also refer to pulleys that have an adjustable operating diameter.

By rotating the components of an adjustable sheave an operator can screw (tighten or loosen basically) the parts closer together or further apart. Doing this changes the distance between the two halves and allows the belt to ride higher or lower in the channel or groove. You can see the threads that permit this in the photo to the right. If you can fine tune the size, you can prolong belt life too.

The adjustments are constrained to a specific range, i.e. adjustable sheaves have minimum openings. If you get one that doesn’t screw down far enough to accommodate the width of the belt, well, you’re screwed lol. You’ll have to order the proper replacement.

Adjustable sheaves work because they are built from a couple, separate pieces.

There are two main parts of adjustable sheaves that can be screwed or dialed closer together or farther apart. Doing this changes the channel width or the operational diameter. You have one female piece and one male piece.

Then there are set screws that you tighten down, setting and fixing the width. The set screws keep the operational diameter of the sheave from changing later due to vibration or operation, or from falling off altogether. Be sure to tighten the set screws onto the flat surfaces. These faces are flat for a reason.

Changing Fan Speed With An Adjustable Sheave

Adjustable sheaves are commonly used to change fan speed in light commercial air balancing applications. But how is it done? That is what this section is about. This section will also try and explain how to change the dimensions of a sheave. If you can follow along and become a pro, you can figure out ahead of time what the change in fan speed will be.

Keep in mind that this article appears somewhere else, need to give the proper credit. Here is the link to the original article on how to use an adjustable sheave to change fan speed.

Adjustable sheaves are more expensive than fixed ones, and motor sheaves are smaller than fan sheaves, adjustable sheaves are generally used on the motor shaft and not on the fan shaft.

How They Work

adjusting a sheave

The two halves of an adjustable sheave

We know how they work, we just talked about it a few paragraphs up but…

As the two sections of an adjustable sheave are turned closer to each other, the belt is forced to the outside of the sheave. This requires the belt to have to travel a greater distance around the sheave.

This increases the speed of the fan.

And then the opposite would be true too yeah? If we unscrewed the sheave sections, the belt would need to travel less around the groove…

Sheave Measurement

In order to measure the pitch diameter, where exactly the belt rides on a sheave, you may or may not have to remove the sheave from the shaft. Using calipers, measure the outside diameter of the belt, while it’s wrapped around the sheave. The diameter of the sheave is the actual outside dimension of the sheave. These are usually two different diameters or values.

Screw in or out the sheave in order to increase or decrease the diameter to the calculated size. Now install the belt around the sheave again. Measure the diameter to verify that the right pitch has been dialed in.

After you’ve measured and adjusted the diameter to the required size, re-install the sheave onto the shaft. You can use an air flow hood to verify that the air flow you expected is being achieved.

Fan Laws

Fan laws are equations or formulas that allow you to calculate belt and sheave changes prior to making them. This saves you time and the hassle of having to make a minor adjustment and measuring again. If you are able to do the math right ha ha.

Using the first fan law, you are able to calculate the change in sheave diameter you would need in order to increase or decrease fan speed to make the fan deliver your desired, or required, air flow.

1) Start by measuring the air flow at the fan.

  • Fan air flow can also be plotted by measuring the fan speed in RPM and the fan operating total external static pressure, and by then plotting the fan air flow in the manufacturer’s fan performance tables.
how do you adjust a fan sheave

The two pieces of an adjustable sheave side by side.

2) Measure the outside diameter of the belt riding on the sheave.

  • Also measure the physical outside diameter of the sheave just to be sure it is large enough to adjust to the required diameter.

Let’s use this example.

You have a 3″ adjustable motor sheave and the fan delivers 2300 cfm on a system that demands 3000 cfm. Here is the formula with our data:

First, divide 3000 cfm (desired) by 2300 cfm (current) to find the ratio of air flow increase.

3000/2300 = 1.3

This shows us that we are 30% “off” when comparing our required air flow vs. what the fan is actually delivering. So, the air flow needs to increase by 30% for the fan to deliver the required air flow.

Now we multiply the original sheave belt diameter of 3″ times 1.3 to find our new sheave belt diameter of 3.9″.

Now you can adjust the motor sheave pitch to 3.9″ to raise the air flow to out desired set point of 3000 cfm. This fan law will only work if the sheave diameter is at least 3.9″ or so.

Keep in mind that you can’t just go around your building or property and starting increasing the air flow. In our example, we would want to make certain that both the fan and motor have the capacity or rating to now handle 3000 cfm.

Look at how our fan air flow increased by 30% and sheave size also increased by 30%.



Multiple Groove Sheaves
2 groove sheave

A sheave with places for two belts. Snazzy.

Multiple groove sheaves are just like they sound. Instead of having just the one channel for a belt to ride in, they might have a few. Having more channels allows for more belts, more even wear, more efficient transmission of power, and also provides some form of minor back up in case say 1 of 3 belts frays and gets thrown.

Multiple groove sheaves are generally seen on larger motors and pieces of equipment. When changing out the belt on a sheave with multiple channels, it sometimes helps to move the belt one channel on each end at a time. Watch your flipping fingers too on these types of sheaves. It is very easy to not pay attention and get a finger caught in one of the channels.

Sheave Alignment

It is oh so important that you align the motor and fan sheaves properly. And let me tell ya, I suck at this part. I’ll install the motor, shaft, sheaves but would prefer to leave the alignment to someone else. Why is the alignment of sheaves so crucial?

When you mount sheaves to any shaft, you absolutely must align both sheaves to each other. Why? By having both sheaves in almost perfect alignment will increase power transmission and reduce wear and tear on the belts, sheaves, bearings, and motors. For even better efficiency and prolonged equipment life, you can also make sure that both the motor and fan shafts are aligned with each other.

Alignment can be done on smaller fans by simply laying a ruler or straightedge flat on the face of each
pulley to assure both pulleys are in line with each other. Or you can use lasers, dial indicators, etc.

Calculating Motor Sheave Pitch Diameter
changing fan speed with sheaves

Here is a multiple groove sheave installed on an AHU for better power transmission.

This last section should be brief. It will be a brief explanation on how you can figure out motor sheave pitch diameter. This also sort of goes along with using an adjustable sheave to change fan speed above. In fact, this may just be a different way to state the same fan law too.

To calculate motor sheave pitch diameter all you have to do is:

  • Multiply the driven sheave rpm by the driven sheave pitch diameter and then divide by the motor sheave rpm.

To calculate the driven sheave pitch diameter, you:

  • Multiply the motor sheave rpm by the motor sheave pitch diameter and then divide by the driven sheave rpm.

Equation:

  • Motor Sheave Diameter X Motor Sheave rpm = Driven Sheave Diameter X Driven Sheave rpm.

Conclusion

This post was brought to you by sheaves. And the letter S.

We went over what sheaves are and how they are used. We also described the difference between fixed and adjustable sheaves as well as how to calculate motor sheave pitch diameter and how to change the speed of a fan by altering your adjustable sheave.

There was quite a bit of information in this post and between you and me, I had no idea that sheaves were this involved. I thought this would be like a 30 minute post. Turns out, I didn’t know very much and I sure as heck learned a ton by simply putting this together.

If you have any questions about sheaves, let’s do our best to get them answered. If you have a comment, feel free to get in touch. Talk with you soon.


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