Timing Pulleys and Belts

The 18x24, 25x25 and 24x48 machines use timing belts and pulleys.

These components are standard sizes that can be purchased from a variety of suppliers including mcmaster.com and sdp-si.com.

The suppliers will list the pitch, number of grooves or teeth, material, belt width, bore size, bore configuration, flange configuration, pitch diameter, outside diameter and length of the pulley and belt.
This list of options can over-complicate a simple task. Some of these values can be ignored while others are important.

The plans give the pitch, number of teeth, bore, and belt width and length.

The following information should simplify the purchasing process. It also will help with the trade-offs that may be necessary when the desired options are unavailable from a given supplier.
Pulley and Belt Pitch


The 18x24, 25x25 and 24x48 machines use XL pitch components.

XL pulleys and belts have a pitch of 1/5 (0.2) inch. They have five teeth per inch.

Choosing XL will automatically choose 0.2 pitch since this is part of the definition of XL belts and pulleys.
Therefore, it is not necessary to search for belts and pulleys by any pitch attributes other than XL.

Teeth or Grooves, and Pitch Diameter

Timing pulleys and belts can be purchased with a different number of teeth or grooves. Some suppliers use the term "groove", others say "teeth"; the value is the same. The plans use the term "teeth."

The number of teeth will automatically determine the pitch diameter of the pulley. Therefore, when choosing a pulley's size by the number of teeth, the pitch diameter can be ignored.

Some of the suppliers' listings give an incorrect value for the pitch diameter of the pulleys.
This typo can cause confusion; again, as long as the number of teeth is correct, the pitch diameter will be a given value, and the pitch diameter value can be ignored.

Pitch diameter is not mentioned in the plans because it does not matter in terms of building the machines, only in the design. The number of pulley teeth is given in the plans.

Pulley teeth is a simple count-able value that automatically takes pitch diameter into consideration.

Material, Bore, and Bore Configuration

Pulleys are available in a variety of materials including solid plastic, plastic with an aluminum or brass insert, plastic with an aluminum tee insert, solid aluminum, and solid steel.
Larger bore pulleys are only available in solid metal.
The bore is simply the hole in the pulley for the stepper shaft, axle or leadscrew. This of course has to match the given shaft.
The steppers' shafts are usually ¼ (0.25) inch. The leadscrews will vary.
Aluminum flanged pulley with no hub
No-hub aluminum pulley.

Above is an aluminum pulley without a hub. Note that the set screw is in the toothed section of the pulley.

Plastic pulley with Tee insert
Tee insert plastic pulley.

The above image is a plastic pulley with a Tee insert. A removed insert is in front to show the Tee shape.

Hubbed and hub-less XL pulleys with aluminum insert
Plastic with aluminum insert.

The two black pulleys above are plastic with aluminum inserts. The left pulley is hubbed, the right is hub-less.
The hub is the projection from the side of the pulley around the bore.

The advantage of a hubbed pulley is the set screw is usually in the hub; therefore, the screw can be reached with the belt on the pulley.

Hub-less pulleys are narrower than hubbed pulleys.
This can be handy when clearances are tight, but it can be a problem when the pulley needs to extend beyond the end of the stepper shaft, for example.

The pulley length value can be ignored. Again, the hubbed pulleys will be longer, and choosing hub or no hub will automatically choose a length.

The plastic with aluminum insert XL pulleys have held up as well as the solid aluminum pulleys in this shop.

The set screws in solid plastic pulleys can strip the plastic, so a metal hub or insert is recommended. Otherwise, shopping by price and availability has served well.

Flange Configuration

Flanges keep the belt from falling off of the pulleys. Derailments can happen for no apparent reason; therefore, all pulleys should be flanged if possible.
Some larger pulleys are not available with flanges. A single flange is better than no flange.

Two flanges with hub is the first choice. A pair of flanges is more important than a hub, if the choice between the two has to be made.

Pulley Belt Width

All XL pulleys in the plans are for belts that are 3/8 inch wide.

The default settings for the suppliers are different. Be sure to set this width value in the shopping parameters. The default setting can reset with page refreshes; take care.

Outside Diameter

The suppliers' listings may give a choice of diameters.
These values can refer to the size of the flange on the pulley, not the pitch diameter.
The flanges can be a variety of sizes. The range of sizes is small and will not affect the performance of these machines. This flange diameter can be ignored when choosing a pulley for these machines.

Timing Belts

The belts are listed by pitch, width, and length.
All XL belts in the machines are 3/8 inch wide.

The number of teeth on the belt will determine its length, just as the number of teeth on a pulley will determine its diameter.

The plans give belt size values in terms of length or number of teeth or both.
Again, one value will mandate the other value, and both values are listed by the vendors.

All XL belts specified in the 18x24, 25x25 and 24x48 plans are closed ended; this means they are a loop like a fan belt.
All belts used on the machines are single sided; they only have teeth on one side.

The belts are available in a variety of materials and tension members or reinforcement cords. Each has its advantages and disadvantages.

For these machines, shopping by price has served well. The performance differences between the different belts on these machines have proved to be virtually nonexistent.

Some suppliers only sell one type of belt, which usually has fiberglass cords. These machines use larger idlers to help lessen the stress on the fiberglass cords that results from excess flexing.

Enlarging Pulleys' Bores

The bores of aluminum pulleys can be drilled over-sized. This can be handy when the proper sized bore is out of stock, or a small leadscrew is upgraded to a larger one.

When drilling-out the bore, incrementally step up the drill bit sizes to help ensure that the hole remains centered in the pulley. Remove the set screws before drilling.

When a vise and/or drill press are unavailable, wrap the teeth of the pulley with a number of layers of tape, and clamp the pulley with Vise Grips or similar, and drill the bore. Take care not to scuff the pulley's teeth. Moderately damaged teeth can be cleaned with a file.

There may not be enough hub left for the set screw to grab. Drill and tap a hole through the toothed section of the pulley for the set screw.

Note: 12mm is a standard size of leadscrew found outside of the US. Pulleys with this size bore can be hard to find in XL pitch.
Drilling-out the bore of an aluminum imperial sized XL pulley will work.

The belt and the ¼" bore stepper pulley can remain the imperial part numbers listed in the plans.

Keeping Pulleys Tight

Pulleys' set screws can work loose, which will cause backlash.

A tiny amount of play can cause a significant amount of lash. The evidence of this lash can be out of round circles or uneven corners in the work.

The cause can be hard to find when not looking specifically at the pulley and its fit on the stepper or leadscrew. The set screw can hold tightly enough to feel as though it is tight, but it can be just loose enough to slip a fraction. The slipping can be too small to feel by turning the pulley by hand when parts are assembled.

The pulleys often have tiny set screws that are very difficult to tighten. A solution is to tap the set screw's holes to a larger size. #10-32 has worked well in this shop. #8-32 has also worked, but the larger #10 screws are easier to tighten.

When installing the pulleys on shafts that have a flattened surface, as found on some steppers, wiggle the pulley as the set screw is tightened. This will help ensure that the set screw is well seated onto the flat.
It is easy to tighten the set screw at an angle to the flat, which will allow the set screw to work free and cause the problems just described.

Many pulleys only have one set screw. Drilling and tapping a hole for a second set screw can help. The second set screw is usually positioned 90 degrees from the original.

Loctite thread-locker used between the pulley and the shaft will also secure the parts, but it is extremely difficult to remove a pulley once Loctite has been applied. In this shop larger set screws are installed before thread-locker is used.
The 10x9 and 13x13 are direct drive. The leadscrews are directly coupled to the steppers without belts and pulleys.

The 18x24 uses belts with pulleys on the X and Y axes.
The 25x25 machine only uses a belt with pulleys on the chassis axis; its other axes are direct drive.

The 24x48 uses belts and pulleys on all axes for all configurations.