How to Support and Capture a Shaft
The VEX EDR system uses square shafts rather than round axles to allow assemblies to spin or rotate. This square shape allows the shafts to fit into a square socket in the motors and provides a physical connection to drive shafts, instead of a friction connection which is used with round axles. The square shape of the shafts also allows the motion components such as Wheels, Gears, and Sprockets, as well as, sensors like the Optical Shaft Encoder and the Potentiometer to have a physical drive connection to spin them. This is because they have a molded plastic square hole or a metal insert with a square hole sized to allow the shafts to slide through the components. These shafts are also known as drive shafts.
The Shafts are available in a number of different lengths up to 12” and come in two versions, the Shaft and the High Strength Shaft.
These shafts are ⅛” square bar. Shafts have been part of the VEX EDR system since its introduction and are compatible with all VEX Motion Products. The shafts can be cut to custom lengths, however they can twist, bend, or shear off when subjected to extreme stress.
High Strength Shafts
These shafts are ¼” square bar. The High Strength Shafts were introduced later in the VEX EDR product line and they are only compatible with VEX Motion Products which have been designed to accommodate the larger shaft such as the V5 Smart Motor, the High Strength Gears, the High Strength Sprockets and Chain, and the 3.25” Wheels. The shafts can be cut to custom lengths and can withstand high stress, however they will not pass through the square holes of structural metal and require a custom hole to be drilled (5/16” or 8mm drill bit) if this requirement is part of an assembly design. The High Strength Shafts weigh more than the standard Shafts.
|Shaft||High Strength Shaft|
The holes in structural metal are square so it is essential to support a shaft in order to have the shaft spin smoothly. In addition, in nearly all assemblies it is important to provide at least two parallel points of support. If two supports are not provided for each shaft, the shaft will be allowed to slightly pivot up and down on the single point of support and it will make the shaft harder to spin. The heavier the robot assembly the shaft is supporting, the more important it is to provide these two points of support. There are a number of parts available to provide these supports.
The parts available to support the ⅛” square shafts include:
- Bearing Flats are plastic parts with a series of threes holes which will support a shaft in 1 hole and allows Screws and Nuts or Attachment Rivets to be used for mounting in the remaining holes. The Bearing Flat has plastic prongs on one side designed to be inserted into the metal’s square holes. If these prongs are missing, the bearing should be discarded because the bearing can loosen under stress. The Bearing Flat is designed to have all 3 holes backed by structural metal.
- Pillow Block Bearings are plastic pillow blocks which allows a shaft to be offset either above, below, or to the side of the structural metal piece the bearing is mounted on. Pillow Block Bearings are not sold separately. They can be purchased in the Pillow Block Bearing & Lock Bar Pack.
- 1-Post Hex Nut Retainer w/ Bearing Flats are quick assembly plastic bearings. One end of the retainer contains a post which is sized and shaped to securely fit into the square hole of the structural metal piece. The center hole of the retainer is sized and slotted to securely fit a hex nut, allowing a #8-32 screw to be tightened without the need of a wrench to hold the nut. The other end of the retainer has a hole for a Shaft to pass through. The 1-Post Hex Nut Retainer is designed to be backed by structural metal.
High Strength Shaft Bearings
The only part available to support the High Strength Shaft is the High Strength Shaft Bearing. These are plastic parts similar to the Bearing Flats with the exception that the center hole is sized for the ¼” shaft to pass through. High Strength Shaft Bearing can be used by selecting a High Strength Shaft which is the exact length to be sandwiched between two bearings attached between two pieces of structural metal. However, in this configuration, the shaft will not have passed through the structural metal and will only be supported by the plastic of the bearing which may fail when placed under stress. Otherwise, a custom hole needs to be drilled through the structural metal large enough to allow the High Strength Shaft to spin freely after it has been inserted through the bearing and metal. The High Strength Shaft Bearing is designed to have all 3 holes backed by structural metal.
|Bearing Flats||Pillow Block Bearings||1-Post Hex Nut Retainers||High Strength Shaft Bearings|
Examples of Two Points of Support
|1 Point of Support (Poor)||2 Points of Support (Good)||2 Points of Support (Good)|
Shafts need to be captured in order to have the shaft spin freely and still not slide out of the assemble. In addition, parts which the shaft is intended to spin need to be captured and/or fixed to the shaft so they will spin with the shaft.
There are some part options to capture ⅛” shafts. When capturing a shaft placed into a motor socket, it is important to fix a collar in an orientation such that it comes up against a structural component opposite from the motor. This will securely keep the shaft placed in the motor. It is also important to fix wheels, gears, and sprockets from sliding back and forth on a shaft. One way to do this is to fix a collar to the shaft on each side of the component. When collars are placed against a piece of structural metal or a bearing, placing a washer between the part and the component can reduce friction.
The following parts can be used to capture an ⅛” shaft:
- Clamping Shaft Collars are plastic cylinders with a ⅛” square hole running through the center of the cylinder, the cylinder has a longitudinal split down its length with a perpendicular screw hole for a #8-32 Screw to pass through and be secured with a Nylock Nut. These collars are designed so the shaft is inserted through the square hole and then the #8-32 screw is tightened to clamp the collar to the shaft. The Clamping Shaft Collar is the widest of the parts used to capture shafts, space and shaft length needs to be designed into the assembly in order to accommodate this width.
- Rubber Shaft Collars are a thin rubber cylinder with a ⅛” square insert running through the center of the cylinder. These collars are designed so the shaft is inserted through the square and the collar captures the shaft by the friction applied. The Rubber Shaft Collars are the thinnest of the parts used to capture shafts, however, they do not have a clamping screw applying pressure to the shaft like the Clamping Shaft Collars, or a set screw which has been tightened against the shaft like the Shaft Collars.
- Shaft Collars are metal cylinders with a round hole running through the center, sized so an ⅛” shaft can slide through the hole. On the side of the collar is a perpendicular #8-32 threaded hole with a set screw in it. These collars are designed to have the set screw tightened against the shaft. The set screws require a 5/64” Hex Key to tighten the screw. The #8-32 threaded hole in the collar allows for the set screw to be removed and either a #8-32 Screw or a #8-32 Couplers for Standoffs to be substituted. These substitutions can lead to some creative assemblies. The set screw in the collar can create a bur in the shaft if tightened too much. If this happens, the bur should be filed smooth before the shaft is re-used.
- High Strength Shaft Collars are the only parts available to capture a High Strength Shaft unless the shaft is captured by sandwiching it between two bearings backed by structural metal as described above. The High Strength Shaft Collar is nearly identical to plastic Clamping Shaft collar, with the exception of the collar has a ¼” square hole running through the center of it.
|Clamping Shaft Collar||Rubber Shaft Collar||Shaft Collar||High Strength Shaft Collar|
Fixing Components To Shafts
Another type of shaft capture is to fix a component to the shaft so it will spin with the shaft. Many components can be easily fixed to the shaft because they have either a molded square hole or a metal insert with a square hole as mentioned above. When a metal component needs to be fixed to a shaft an additional part is needed. For the ⅛” shaft these parts include:
- Drive Shaft Lock Bar is a metal plate which has two end holes sized for a #8-32 Screw and attaching Nut to mount the bar to a metal component and three ⅛” square holes, which allows a shaft to be inserted through the middle square hole allowing the shaft to drive the metal component to spin. Note, the two squared holes on the sides of the center square hole can be used with the discontinued ¼” pitch metal.
- Lock bar is a plastic bar which has two end holes sized for a #8-32 Screw and attaching nut to mount the bar to a metal component and there is a metal insert with an ⅛” square in the center which allows the shaft to be inserted through the square hole allowing the shaft to drive the metal component to spin. The metal insert is inserted into a slot in the plastic bar which has a ratchet tooth design allowing the metal insert to be removed and then reinserted into the slot with the square hole oriented in a different position. It is essential that the lock bar is mounted on the metal component so it is orientated with the metal insert side of the bar flush against the metal, otherwise the shaft can push out the metal insert from the bar’s slot releasing the metal component from the Shaft. Lock Bars are not sold separately. They can be purchased in the Pillow Block Bearing & Lock Bar Pack. The plastic Lock Bars are more prone to fail under extreme stress than the metal Drive Shaft Lock Bars.
At the time of the writing of this article, there are no available lock bars from VEX for the High Strength Shafts. Nonetheless, a custom hole can be drilled through the metal structural component and a High Strength Gear or a High Strength Sprocket with mounting holes can be mounted to the metal. Then, the High Strength Shaft can be inserted through metal component and gear/sprocket. The mounted gear/sprocket acts like a lock bar allowing the shaft to drive the metal component to spin. If there are specialized tools available one of the holes in a metal plate or a metal bar can be enlarged to a ¼” square hole with a broach to make a custom lock plate for the large shaft. Presently, there is an after-market vendor which sells VEX metal bar of different lengths with a hole broached out to a ¼” square.
|Drive Shaft Lock Bar||Lock Bars||Metal Bar With a ¼” hole broached|
Examples of Shaft Capture
- Rubber Shaft Collar captures Shaft from sliding out of the drive socket in the motor
- Two Rubber Shaft Collars captures the 4” Wheel from sliding back and forth on the Shaft
- Square Metal Inserts in the 4” Wheel fixes the wheel to the Shaft
Free Spinning Assembly
There is a circumstance when a gear, sprocket, metal component, or wheel may not be desired to be fixed to a shaft. In this case, the component (wheel or metal structure) would have mounting holes and be mounted to the driven or “output” side of a gear or sprocket drive system. The driven gear or driven sprocket and wheel can use Round-Hole Inserts to allow them to free spin on the shaft, The wheel or metal component mounted to the driven gear/sprocket will spin from the driving (input) gear or driving sprocket and chain and not by the shaft it is inserted on. The free spinning assembly does not have the additional friction of spinning both the shaft and assemble.
An example of a free spinning assembly
- Driven High Strength Gear and 3.25 Wheel both have Round-Hole Inserts to allow them to free-spin on the shaft. The 3.25 Wheel is mounted to High Strength Gear.
- The driving High Strength Gear has Metal Square Inserts to fix it to the shaft and allows the motor to drive the shaft and in turn drive the gear, spinning it.
- The Shaft does not have to spin with the wheel assembly.
Supporting and capturing Shafts is an important aspect of assembling a robot. Whether it is a classroom game or an elimination match in a major robotics competition, there are not many things as disheartening as looking out on the field and seeing a gear, wheel, or sprocket from the robot laying on the ground or moving a joystick on the robot’s controller and nothing happens because a shaft has slid out of a motor.
| Safety Hazard:
Be sure to keep fingers, clothing, wires, and other objects from getting caught between moving components.
Shafts and hardware can be purchased at https://www.vexrobotics.com/vexedr/products/motion.