Secure plate for rotational mount

[RobDooley]

I have a design for a rotational joint that is coming loose. The design is a steel post and top plate secured with flat head screws. The stack up of the rotating unit and thrust washers are the same height as the post in the largest tolerance. When the assembler tightens the flat heads to 14in-lbs they apear to be fully seated. Using the torque driver we are able to go around 5 times before they are do not rotate.

The flat head screws are steel M4 with a nylok patch. The post is a machined steel.

Any ideas on testing to find out what is going on.

Thanks Rob

 

[MintJulep]

Embedding and relaxation.

 

[drawoh]

RobDooley,

How much force are you putting on the post, and what sort of leverage does it have against the screws?

14lb.in is not a bad torque for non-locking M4 screws of soft steel. The locking mechanism uses up more tightening torque than standard fasteners, reducing your clamping force.

Are there any soft materials in your structure?

JHG

 

[hydtools]

My guess is that all the screws are not seating when assembled. It is very unlikely you will get more than one flathead screw to fully seat in a multiple set of screws unless each countersink seat and tapped hole are match drilled.
I prefer hex socket capscrews in counterbores for multiple screw attachment where the screw heads must be below the surface. The screws will all seat all the time.

Ted

 

[RobDooley]

drawoh,

The post is a upside down t with a plate on the top sandwiching a cantalivered control panel with thrust washers top and bottom. It appears that the plate is fully seated on the top of hte post. The thrust washers are Igus polymer bearings. They are softer than the structure.

The forces generated at the post are about 200lbs. The plate is secured with 8 M4 flatheads.

Thx Rob

 

[RobDooley]

Hydtools,

That is one of the thoughts that have been raised by other here. I am trying to reuse parts from a previous design.

thx Rob

 

[dvd]

What is coming loose?

I have had experiences with countersunk holes having bad surface finish in the countersink. The constant fluctuation in forces causes the surface of the countersink to burnish and thus the length through bore of the fastened parts can change and take pre-load off of the screw. If this is the case, either design a counterbore, or put a surface finish callout on the countersink, or re-design the parts the way you know they should be.

 

[drawoh]

RobDooley,

How big is the pitch circle for your screws?

Do the polymer thrust washers affect the length and strain of your screws?

JHG