Need advice on keeping motor screws from getting loose

I’m curious what techniques can be used to keep motor screws (and other screws/nuts/shaft collars for that matter) from getting loose. We don’t like to overtighten because that strips our screws and hex screwdrivers. On the other hand we’re weary of all the time spend retightening loose screws/nuts and collars. Thanks!

For motor screws we usually use some Loctite, works well for screws you don’t want to come out most of the time, and if you do, it is still usually possible. (Only wont be if you use an excessive amount or get a very strong kind.)


or you can go to the hardware store and buy some good quality allen keys (they NEVER strip!)
and you can also get some better quality screws too (never strip also)
here we have a saying: “is that a vex screw? toss it, it’ll strip and make this impossible to disassemble)”

and we over tighten our screws to prevent them falling out of important places, we must have a pair of pliers when tightening to help with the leverage!

We try to design our robot so that the motors can be easily tightened and replaced. After every match everyone looks over the robot and tightens all of the loose screws. This has worked out pretty well for us.

Step 1: Design - the robots need to be designed to stay together and access to critical components. You need to have three screws at each corner of the frame, one on each face. The triangle mount means security.

Axles need to be supported and bearing blocks used. On high stress axle setups (like driving wheels) two collars will hold the load, but four will spread the load.

Cross struts and braces make for frames and supports that don’t move or bend. The triangle is always your friend.

Step 2: Assembly - use the nylon lock nuts for parts that you don’t want to come apart and are in areas hard to get to. Use of the #8 lock screws with the Kepts nuts gives a very secure assembly.

Be methodical and check each nut/bolt. I can’t tell you the number of times I’ve looked at robot and seen loose screws just after a “we checked everything” session.

Step 3: Tools - Use the open ended wrench to tighten nuts. When the tips of the allen keys become worn, file or grind them down. We use high quality T handles and even they become worn after awhile. We also use nut drivers to reach into spaces that the wrench won’t fit into.

I use the “vanilla” Vex Keps Nuts and Vex screws. I tighten them with handy little Sears Craftsman allen-tipped “screwdrivers”.

I remember one of the things I learned in my semester of Mechanical Engineering: Metals (Vex screws included) will stretch a bit (aka plastic deformation) when tightened past finger-tight (when put under tension); and if they aren’t stretched too far they will return to their original length when loosened. In bolts and screws this stretching puts the parts under tension and works with washers to keep the joint securely fastened.

However, there is a limit to this stretching. So, when I tighten the connections for critical components, I try to feel when the screw (and Keps washer, and …) starts to strech just a bit, and then I stop.

Also, I use C-channel everywhere I can and I create rigid structures that are full of solidly built rectangles.

When the framing starts to slide at a joint, the screws/nuts loosen in a heartbeat. So, build structures that aren’t going to have wigglely corners.

Following those guidelines I have built robots that I carry around for weeks or months with almost no loose screw problems - all without the hassles of using Locktite or locknuts.


This is practically the single most obvious difference between rookies and experienced VEX builders. Experienced builders make robots that don’t lose fasteners and have hands covered with little calluses from gripping Allen wrenches very, very tightly. :slight_smile:

Another tool tip – use good quality Allen wrenches. Before the VEX advanced toolkit came out, we stocked up on hard-chrome wrenches from Wiha Tools. They are more expensive, but we have some that are two years and have not rounded off. They also sell ball-end wrenches that are handy for getting into the odd corners of a robot.

We often use sawed-off Allen keys for getting into tight spaces, as well as the ball-end wrenches Rick mentioned, for tightening at an angle.