Timing Pulleys and BeltsThe dual leadscrew, rack and pinion, and belt drive machines use timing belts and pulleys.These components are standard sizes that can be purchased from a variety of suppliers that are listed in the plans. The plans give the pitch, number of teeth, bore, and belt width and length. 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 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. .................
Pitch All of the machines
in these plans that use pulleys and belts, use XL pitch, with the
exception of the belt drive machine, which also uses L pitch.XL pulleys and belts have a pitch of 1/5 (0.2) inch. They have five teeth per inch. L pulleys and belts are 3/8 (0.375) inch pitch. Choosing XL will automatically choose 0.2 pitch since this is part of the definition of XL belts and pulleys. The same is true for L at 0.375 inch pitch. Therefore, it is not necessary to search for belts and pulleys by any pitch attributes other than XL or L. 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
ConfigurationPulleys 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 are 1/4 (0.25) inch. The leadscrews will vary. .................
On the right is an aluminum pulley without a hub. Note that the set
screw is in the toothed section of the pulley. The second image is a plastic pulley with a Tee insert. A removed insert is shown in front to show the Tee shape. The third image is a pair of plastic pulleys 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. 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. The plastic with aluminum hub pulleys have held up as well as the solid aluminum pulleys in this shop. On odd occasions, there have been problems with the flanges working free on the aluminum pulleys, though this should not happen when parts are well aligned. 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. Belt Width All XL pulleys in the plans are for belts that are 3/8 inch wide. All L pulleys in the plans are for belts that are 3/4 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. Flange Configuration Flanges keep the belt from falling off of the pulleys. Derailments can happen out of the blue 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. 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. Belts The belts are listed by pitch, XL or L, width, and length. All XL belts in the machines are 3/8 inch wide, all L belts in the machines are 3/4 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 vendor. All belts used on the machines are single sided, they only have teeth on one side. All XL belts specified in the plans are closed ended; this means they are a loop like a fan belt. The L belts used in the belt drive machine are all open ended. But they can be bought as open or closed, they will be cut if closed, so it does not matter which is purchased. It can be less costly to buy a closed belt than a similarly sized open belt. Go figure. Again, shopping by price serves well. 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 leadscrews are a standard size 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 1/4" bore stepper pulley can remain the Imperial part numbers listed in the plans. .................
The machines, which use pulleys, all use pulleys with the same number
of teeth on
the X and Y axes to facilitate upgrading. This way, the belt lengths,
and stepper and idler positions will not have to be altered if/when the
leadscrews are replaced.
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The 13 x 13 is all direct drive. The
leadscrews are directly coupled to
the stepper without belts and pulleys. The 25 x
25 machine only uses a
belt
with pulleys on the chassis axis; its
other axes are direct drive.The rack/leadscrew uses belts and pulleys on all axes for all configurations. |