Proper motor attachment

Is there a place I can find something in writing which shows the proper way to attach a motor? I have a student who insists on using plastic spacers in lieu of a bearing flat because he can’t get the bearing flat to fit without cutting the axle (which I hate to have him do - that’s another question; do people just keep shaving those down when they need them to fit precisely because they are using one motor on either side of the axle for double the power?). He won’t take my word that the bearing flat is necessary for the life of the motor to prevent that sideways torque from burning out the internal bearings…

From what I’ve found, as long as the axle is supported at two points (both motors in this case), using a bearing is unnecessary and not having one won’t reduce motor life. However saying that having a bearing certainty won’t hurt and will likely keep the system a tad bit more reliable. Typically your going to have to cut the axles to a precise amount when doubling motors like that, there isn’t any way to get around it.

I am not an expert, but have been using Vex since AUG-2006…

The bearing flat is to support the Axle, to keep the Flexing to a minimum. Otherwise that motion is transferred to the Motor’s Gears and wears them out faster…

I have cut some of my 12" shafts to shorter lengths, but that should not always be needed… You can order just the 12" Axles, and I have, just so I could make Custom Lengths… If you find you keep building the same kind of mechanisms, you can used those Custom Axles over and over again…

When I have used Two Motors, on the same shaft, turning opposite directions, for more Torque, I have used Spacers on the Motors, to Increase their Distance apart, rather than change the Shaft Length.

You might not have the space necessary to do that in this case, but I try everything I can before permanently modifying my parts…

This is my third year using the VEX parts for robotics and I can say that first off, I and my team has cut our fair share of axels. It has always been my understanding that this has been the norm, maybe not though.

I really don’t see how you could use a plastic spacing in the spot of a bearing… The use of a bearing is to create a “proper” fitting for an axel so you aren’t turing a square object in a square hole or rubbing metal against metal (reduced friction); I just can’t see how any sort of spacer could replace that function.

Hope this helps!

Have a look at pg 6 of the clawbot instructions - you can see how the motors are mounted on the drivetrain there.

I tend not to use delrin bearing flats where a motor is attached unless there is a decent amount of weight on that shaft. I’ve found the bearings within the motors to be good enough to support gears etc, if it was supporting a wheel or something I would put the bearing on. Also, if you’re using the shaft couplers then I would definitely put a bearing on because they are much too wobbly otherwise.

It is often necessary to support a shaft in more than two places for structural reasons but whenever you do you have to be careful to line the parts up exactly. Many people mount the bearing on the motor screws but on the other side of the metal. If you do this, the square inserts that lock into the holes in the metal get crushed by the brass inserts on the motor, and so the bearing can get out of alignment, causing friction. In the instructions linked above, one hole is on a motor screw, which is ok. I always mount the bearing in different holes to where the motor is attached if possible. [like jpearman’s post below]

Correct me if I’m wrong but I’m pretty sure that you need 3/4" of shaft beyond the metal to fit into a motor, and a bearing flat is 1/4" thick. So if you want to mount say a 3" shaft between two motors, then that would leave you with 1.5" in between, or 3 holes worth. So you shouldn’t have to cut axles just to do this. Something like this - sorry I don’t have CAD software on hand.


Green=motors, grey=c-channels, black=shaft, scale approximate :stuck_out_tongue:

Of course sometimes you have to use all kinds of different bits and pieces to mount the motor onto depending on your design, and cutting or filing shafts is pretty normal if you need to. My advice is to work out what lengths you are going to use most often if you want to get the most out of your parts.

I think every team creates their own internal standards, we try and do the following.

  1. Always use two bearing flats to support an axle.
  2. Always use lock nuts to secure the bearing flats, no plastic rivets.
  3. Try and always have the screw heads on the outside of the robot, aesthetically better and helps with robot size.
  4. Trim all axles to length, however, we try and avoid cutting the standard length axles (2", 3" and 4") and make custom axles from the 12" lengths. We don’t trim axles until the design is finished if possible in case the design changes. We also try and arrange that axles will be cut to the nearest 1/2 inch where we can.
  5. If at all possible allow access to the motor screws so as to be able to tighten them

Here is an example of what is typically done (it’s not a real part but shows the concept, axle not trimmed here).

[ATTACH]7126[/ATTACH]

[ATTACH]7127[/ATTACH]
motor_ex_1.jpg
motor_ex_2.jpg

A hundred times this. I can’t tell you how many mechanisms we built without thinking about accessibility and maintenance. Sure we would make the motor fit really nicely somewhere, but it was a huge pain to have to tighten the motor screws when the motor became loose (and they always become loose at some point).

What Jpearman is showing is in my opinion a great example of motor attachment. You have the bearing supporting the shaft and a shaft collar inbetween the bearing and the motor housing, ensuring the shaft won’t come out. Plus the motor screws are accessible for an added bonus. Beautiful.

We’re getting a little off topic but I wanted to show another example. I had posted this photo a few weeks back.

https://vexforum.com/attachment.php?attachmentid=6987

Here is the CAD for this (slightly more recent, the design was improved), I have made some of the structure transparent so you can see where the bearing flats are mounted.

[ATTACH]7128[/ATTACH]

The axle under the most stress is the one with the final 12 tooth gear that drives the 60 tooth to which the arm is attached. It has three support points with the third being in between the two gears. This design did not use any structure above or behind the arm as I wanted the arm to be able to rotate more than 180 degrees to the rear of the robot.

Something else I try and do is make the spacing between the C channels in this type of arrangement allow standard structure to be attached. This generally means not using, for example, standoffs directly but adding extra washers and spacers so that the 1/2 inch hole pattern aligns correctly. You can see part of a 5x25 plate and 1x2x1 C channel that connect all the tower supports together. The arm is also not directly attached to the 60 tooth gear but uses spacers to allow both left and right arms to be again connected using standard structure with holes on the 1/2 inch pitch.
motor_ex_3.jpg

Not really… The Thread is Titled, “Proper motor attachment”.

I think we have some very fine examples of that subject, and related Support Structure…

In following the Clawbot® Assembly Guide, you see some examples of building a Robot. The community is providing a few more… :wink:

I wish to thank the OP for bringing up this question… In a desire to prevent unneeded damage to their Robot, Many Best Case examples have been brought forward… I think we all win.