 |
|||
 |
On this page we provide the novice builder with guidelines and information regarding axles and shafts. As always, we try to ensure that the information we provide is accurate but there are no guarantees. It is up to you to ensure that the materials and how they are used will stand up to the rigors of robot combat. |
||
 |
|||
 |
|||
 |
|||
 |
|||
 |
|||
 |
|||
 |
|||
 |
|||
 |
|||
 |
|||
Axles and Shafts Passive and Live Shaft Collars
In technical terms an axle does not rotate it is fixed. Thus wheels, sprockets and gears rotate on the axle. A shaft on the other hand rotates and the hubs, wheels, pulleys etc are attached to the shaft. With that definition out of the way, for our purposes we will use the terms interchangeably.
Axles don’t just carry the wheels of your robot, they
carry the weight of your robot and sometimes the weight of your opponents robot
as well. On occasion the axle may have to withstand the extreme forces
encountered when your robot is tossed into the air and comes crashing down. As
part of the drive system, the axle may have to endure huge torque forces when
drive motors are suddenly and repeatedly slammed from forward into reverse and
back again. The more strength and support you have in axles the better and can
mean the difference between winning, and watching your wheels go flying across
the arena.
A general guideline for selecting axles is as follows. ½”
shafts for 60 lb bots and add ¼” for each weight division above. Thus a Light
weight combat robot would use at least a ½ inch shaft and a middle weight would
use ¾”. A heavy weight would need a 1” shaft and a super heavy weight would
need a 1 ¼”. If you build your bot with exposed wheels or overhung axles
increase shaft diameter by at least 1/8” to ¼”. For spinning weapons use at
least a one inch shaft in bots that are over 100 lbs.
Mild steel shafts are popular amongst bot builders. The
material is relatively inexpensive, readily available and easy to work with.
Another advantage of mild steel is its tendency to bend rather than break. Its
far better to have a robot with a wobbly wheel and still in the fight rather
than sitting helplessly while your opponent lines up for the big kill.
An important consideration in selecting a shaft is the mounting method. To reduce the need for a large heavy shaft, try to design your robot so that the shaft is supported on both ends with the wheel or gear between the supports. This is called a supported shaft.
The other type of shaft layout is called the over hung shaft. Two supports are still needed however one support is at one end of the shaft, the next support is next to the gear, pulley or wheel. The wheel or gear, sprocket or pulley is on the end of the shaft. The advantage to this design is that having the wheels mounted to the outside will make it more difficult for your opponent to flip you over. You also are less likely to become hung up as one of your wheels will remain in contact with the floor and not propped up on the frame or armor. The disadvantage is that this will tend to leave your wheels open for attack.
Passive and Live Axles and Wheels
To move your robot around that means wheels, well….on most robots anyway. So this means something has to turn, either the wheel or the axle. A passive wheel is what you will find on things like wheel barrows and most lawn mowers. The axle is fixed and does not rotate, and the wheel has bearings which allow it to rotate on the axle. To use a use a passive wheel on a robot you will need to provide some means for the drive motor to spin the wheel. This means mounting the sprocket, pulley or gear directly to the hub or rim of the wheel. Exactly how you mount the gear or pulley to the wheel will depend on the configuration and composition of the wheel. You will have to make sure that the gear is centered exactly on the wheel and mounted so that it doesn’t wobble. If the sprocket or pulley is not centered or it wobbles then the chain or belt will alternate between loose and tight as the wheel rotates, which can easily break the chain or cause it to jump off the sprocket.
A live axle requires that the builder provides some means of attaching the wheel to a hub which is attached to the shaft. To handle the additional forces of torque you may need to consider using a larger shaft.
If you use bearings without set screws you will need something to prevent the shaft from sliding. Different types of shaft collars are available that mount on the axle and prevent it from sliding out. The three most common and available shaft collars are:
Single piece with set screw.
Basically a metal donut with a set screw, these are inexpensive however the set
screw can damage the shaft making the collar difficult to remove. If the axle is
damaged by the set screw it can make removing bearings and other components very
difficult.
Single piece clamp-on. Again a metal donut but with a cut on one side. A small bolt is tightened down and this clamps the collar onto the shaft. The advantage is that these collars provide strong clamping power with a small increase in cost.
Two piece clamp-on. A metal donut cut in half and uses two small bolts to tighten it to the shaft. Since the two piece clamp-on collar can be taken apart it makes removal very easy to do without having to remove any components. Besides the advantage of being able to remove the collar easily, these provide strong clamping power, but cost a bit more than the other options.
There are options to using shaft collars. One method is to use retaining rings sometimes call snap rings, or E-clips. The advantage of these is that they are very inexpensive to replace. They can effectively prevent a pulley or gear from sliding on a shaft and weigh virtually nothing. Using retaining rings to hold a gear or sprocket in place requires that a small groove be put into the shaft. This needs to be done precisely and requires the use of a metal lathe. The retaining rings also require the use of special pliers for removing or installing them onto the shaft. This can be made even more challenging if the retaining ring is located in a spot that is confined and difficult to access. Fortunately, retaining ring pliers (AKA Snap ring pliers) are not terribly expensive and can come with interchangeable tips for working in confined spaces. If you use e-clips, you can use a pair of needle nose pliers to put e-clips in place and a blade screwdriver will remove them. However they do not have the same holding power as retaining rings and the problem of removing them in confined spots is still a challenge.
One more option to preventing pulleys or sprockets from moving on a shaft is to install a retaining pin or roll pin. To do this you will need to drill a hole through the hub of the gear or pulley and through the shaft. Then using a small hammer and drift punch the roll pin is set in place. Again the advantage is the light weight compared to shaft collars and the low cost to replace. The disadvantage is that these pins can be sheered off. You will need to ensure that you use roll pins or shear pins that are large enough and strong enough to make sure that the gear or pulley does not come loose. You will also need to make sure that you drill the correct size hole for the pin to go into. If it is to small the pin will be very difficult to install and may possibly damage the shaft. If the hole is to large the pin will not be held securely in place and your will run the risk of having the pin work its way out.
There are other methods of combining sprockets, pulleys and gears onto shafts and axles, such as square, hex and spline shafts. But these methods are not commonly used because the parts can be difficult to obtain and can become quite expensive.