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Recommended Tools for Measuring Pulleys:
Measuring Pulley Outside Diameter
Also Known As OD, length, breadth
Place your pulley face down on a flat surface, then put your ruler or caliper on top, measure from outside edge to outside edge across the circle of the pulley.
The above pulley has an outside diameter of 4.5”
Check carefully to make sure you are measuring across the center. One trick is to hold one side of your ruler still and rotate the outside edge around - the outside diameter is always going to be the longest dimension you measure.
To find a drive pulley based on the Outside Diameter, click FIND A DRIVE PULLEY and use the Outside Diameter filters on the left-hand side of the page to find the right size!
You can also find V-Groove Idler Pulleys the same way by clicking FIND A V-GROOVE IDLER PULLEY.
NOTE: If you have a Flat Idler - you may want to locate the right size based on the Flat Diameter instead of the total Outside Diameter. If you have a flat idler, check out the next section!
Measuring Pulley Flat Diameter
Also Known As: belt diameter, face diameter
You only need to get this measurement if you are using a flat idler pulley.
This is the diameter of the pulley where the belt rides. If your flat idler does not have flanges keeping the belt in place, then the Flat Diameter will be exactly the same as the Outside Diameter.
However, if your pulley has flanges, your flat diameter will be smaller than your outside diameter. If your flat idler does have flanges, you can try and "guesstimate" the flat diameter by measuring the diameter the same as you would the outside diameter, just try to measure as if the pulley does not have flanges.
Looks like this one's about 5"
To be more precise, you can wrap a string tightly around the flat of the pulley where your belt rides. Mark the string where they overlap, then measure its length. Divide that number by 3.14.
In this flat idler, the string measured 10 1/4''. Typing 10.25÷3.14 into a calculator gets you 3.26, so the flat diameter is about 3 1/4”.
No calculator on hand? You can use your browser’s search bar!
Now that you have your idler pulleys Flat Diameter, use that information to filter to the right size Flat Diameter here: FIND A FLAT IDLER PULLEY
Measuring Pulley Width
Measure from side to side across where your belt sits, from outside edge to outside edge. The width dimensions on our site do not include the hub of your pulley.
Also Known As: belt width
This dimension is only needed for measuring flat idlers and drives.
Calipers work best for measuring this dimension.
If you don’t have calipers, you will need to measure the width between flanges from inside edge to inside edge using a ruler and eyeballing it. If your pulley has flanges, the flat width will always be narrower than the overall width. Some flat idlers have a wide angle on their flanges – this helps your belt last longer, but does make it even trickier to measure accurately.
Is this being used as a backside idler? Try also checking the widest part of your belt. If it’s a 5L or B belt, for example, then the flat of your pulley will need to be about 7/8” or bigger.
Left: A ruler is used to measure the flat width.
Center: Even inexpensive calipers can make measuring your parts so much easier.
Far Right: A pulley being used as a backside idler. The flat side of the belt is making contact with the flat surface of the flat idler.
Also Known As: Bore, center hole, bolt size, shaft size.
Measure across the center hole of your pulley. Do not include any keyway in your measurement.
This pulley has an inside diameter of 3/4"
If you know your drive pulley is running on a 1" shaft, or that your idler pulley uses 3/8" bolts, then it's safe to assume that the inside diameter is the same as the shaft or bolt diameter.
To find the pulley with the right inside diameter, click on either the drive pulley or idler pulley link below and then filter to the desired inside diameter on the left-hand side of the product page.
Note: Idler Pulleys often can have their inside diameters easily customized by selecting a bore adapter! For more information on this, contact us at [email protected] or 1-800-776-9315 or visit this helpful blog post: How to Use Multiple Bore Adapter Options
The offset is the distance from the centerline of the pulley to the outside edge of the bore. On a V-groove drive, for instance, the centerline or “middle” of a pulley would be the lowest point of the V, and you would measure from there to the edge of the hub. Generally, the “frontside offset” is the longer of these dimensions, while the “backside offset” is the shorter.
This is tricky to get if you can’t cut your pulley in half, so the good news is you probably won’t need it.
A keyway is a notch made into the hub from the inside hole of a drive pulley and is where your key will be inserted to lock the pulley in place on the shaft. Most keyways are standardized to the shaft your drive pulley goes onto.
Standard Keyway Sizes:
Shaft Size/ Inside Diameter
Standard Keyway Size
1/2” (0.50)
1/8” (0.125)
5/8” (0.625)
3/16” (0.187)
3/4” (0.75)
7/8” (0.875)
1”
¼” (0.25)
1-1/8”
If you have your key on hand, measuring that is certainly the most precise option. Otherwise, a ruler works fine.
In the pulley world, a “pin” is a precisely-cut cylinder that links to the engineering standards of a specific belt type. During the design process, pulleys are “married” to a pin size which will be used in design processes for new equipment.
Did any of that make sense? No? No problem, because all most of us will need to know is the following:
Pin Diameter
Belt Type the Pin is used for
0.3750
3L
0.4375
4L/A
0.5000
4L/A OR 5L/B
0.5625
5L/B
0.7810
C
IMPORTANT: Having a pin diameter of 0.5625" does not mean that a 4L belt will not work, it just means that the pulley was not necessarily designed with that belt type in the mind of the engineer. Check in the “Belts that fit” section to see if your belt is listed on the pulley you are considering.
This is yet another highly technical dimension that you will probably not need to find the correct pulley, but for reference, we've included information on this dimension for you:
If you place two pins on either side of the grooves on a v-belt pulley and measure the distance, you will get the Overpin diameter.
How you can use this, though, is a tool to visualize how far into a groove your belt might ride into the v-belt pulley you’re considering.
For instance, if the Outside Diameter is 7”, but the Overpin Diameter shows 7.35” then the belt is riding at the top of that pulley. Look at another 7” pulley, and the overpin shows 6.5. Now we’re looking at a belt that rides pretty deep into that groove.
Another way to consider this is by looking at different belts inside the same groove:
The diameter of the belt in the groove is also known as the "pitch diameter"
In a 7" pulley with this kind of groove, a 3L belt has a pin diameter of 6", while a 5L belt is at 6.8". If your original 3L belt was riding higher up in the groove of your original 7" pulley, then using a pulley with a 0.5625" pin will give your belt some slack that you might not want.
Trying to determine what type of pulley you need to fix your engine? Maybe you need a specific part to repair a broken washer, fan, or snow blower. Pulleys are a critical component in many household machines and automobiles, but one type or size does not fit all. Choosing the correct pulley type to replace your worn or broken parts can be intimidating if you don’t know where to start. If you feel like your local mechanic is speaking a foreign language, then perhaps we can provide some context on different pulley parts and their purposes to help you find the pulley you need. We will look at drive pulleys, idler pulleys, and driven pulleys to learn more about their differences, similarities, and functions.
What is a drive pulley? You may have guessed it by the name, but drive pulleys are in fact the driving force of a pulley system. The drive pulley connects to the power source, which could be the shaft of an electric motor, or even a hand crank. In turn, the drive pulley then applies a force onto the belt, which causes it to move. It’s important to know what size of drive pulley you need, since the size and shape of the drive pulley in comparison to the driven pulley can control the amount of torque and the speed of the belt. For instance, a larger diameter drive pulley will speed up the driven pulley but provide less torque.
Every drive pulley will be made of steel or some other hard material. There are a couple of defining features on every drive pulley, but they will vary from part to part. You will find a groove around the circumference of the pulley that seats the belt. There should also be some kind of attachment hub to connect the drive pulley to the power source. Drive pulleys may also have a keyway cutout in the center hole. This is another thing you can look for when trying to determine the type of pulley you need to replace.
Where a drive pulley and driven pulley are essential parts found in every pulley system, most pulley systems don’t actually require an idler pulley. Often, larger, more complicated pulley systems will need an idler pulley. If the direction of a belt needs to change from one pulley system to another, that’s where an idler pulley comes in. Sometimes an idler pulley simply holds the timing belt in place, especially if the belt travels for long distances. In this case, the name can be deceiving, since you wouldn’t want your idler pulley to actually be idle! If an idler pulley stops spinning, that is when something can go wrong.
The size and shape of the idler pulley don’t impact the speed or output of the belt, since the pulley is spun by the belt. This might make it easier to find a replacement part, but you can always ask the experts to ensure you’re getting what you need.
Idler pulleys and drive pulleys can look fairly similar, so it may be hard to tell what part you need. A key difference to differentiate the two is to look for a bearing. A functioning idler pulley spins freely on its own. Try spinning the outside of the pulley. If it spins, there is a bearing present, and you know you’re looking at an idler pulley.
Although it may be confused with drive pulleys, driven pulleys essentially provide the opposite purpose to a drive pulley. Where drive pulleys are characterized by their connection to a power source, the driven pulley doesn’t connect to a power source at all. The driven pulley is part of the pulley system, only connected to the drive pulley by a timing belt. The driven pulley complements the drive pulley, working together to move energy from one place to another.
You can often identify a driven pulley based on where it is located. Where a drive pulley can be found on a motor, the driven pulley would be on the other end of the belt, allowing the rotating machinery to operate.
Hopefully, we’ve clarified the differences between pulley types enough that you now know which one you need. But then what? There are still a few more things to consider before making a purchase. As mentioned above, the best place to start is to determine whether you need a drive or driven pulley. From there you’ll need to consider the sizes needed, especially if it’s paramount you keep your machinery operating at the same level as before when replacing a part. Think about any other considerations you might need, such as noise level, corrosion-resistant materials, or pulleys that are made for using ropes or chains instead of belts. Gather a list of specifications you need and work with a reliable manufacturer to get you what you need.
Still stumped about finding the right part? Perhaps your business is out of stock on crucial older pulley parts. That’s where Illinois Pulley & Gear can help. Our custom pulley replacement parts fit the needs of any application.
With over 90 years of combined industry knowledge, we’re the experts at helping all kinds of industries find the right part they need, often faster than the original manufacturer. With the ability to custom manufacture at any time, there is nothing we can’t help you find. Now you won’t have to come back after initially picking up the wrong part, because we’ll leave you in good hands.
If you’re in the Chicagoland area and are lucky enough to catch a worn part before it breaks, we’ll personally come out and recreate the part, so you don’t experience any downtime! That’s what our team does best, providing excellent customer service and custom solutions to our customers every time. Contact us today to learn more about our full selection of custom pulley and gear parts we offer.
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