Rivet for pivoting frame

[westcoastr]

I'm looking for fasteners for an aluminum flat bar frame that needs to be able to open and stay open. to visualize the situation, think scissor lift or convertible car roof structure. the fasteners must maintain a fairly constant clamping force, enough to support about 10 ft-lbs of torque, even after many openings and closings. low cost is important.

I've tried small bolts with lock washers and also lock nuts with nylon inserts, but after a few opening and closings, these all loosened up and the frame would not remain free standing. Pop rivets were slightly better but loosened after a few cycles. I've seen on ladders for example, the spreader bars that lock the ladder open have what appear to be semi-tubular rivets holding the joints together. these allow the bars to pivot open but seem to maintain a constant clamping force. I'm not sure rivets are the ideal fastener as they seem to be better for non-pivoting joints, or ones in which the joint moves freely.

maybe there are some other types that might work? also thinking of trying a conical spring washer (i.e. belleville spring) with a bolt and nut or rivot.

 

[CoryPad]

The applications you describe require shear force resistance, and in fact, work best with no preload (which binds the assembly rather than allowing it to move freely). Rivets are used for these applications. If there are out-of-plane forces, that complicates the matter. My best advice is to design to prevent out-of-plane forces, use a lubricant to allow smooth motion (solid film/graphite/petrolatum if your application cannot tolerate liquids), and use thick rivets that can tolerate some bending.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[unclesyd]

Since we don't have any dimensions here is one type shoulder rivet you might look at.

http://www.hansonrivet.com/w11.htm

Are you could check out something like this binder post.

http://www.hansonrivet.com/w74b.htm

You could use a tubular rivet with a washer.

Hanson has a lot of different close coupled fasteners, look around.

 

[westcoastr]

thanks for some ideas guy. there will be little if any out of plane forces and the little bit of shear is very small compared to the size of the fastener (#10 bolts or about 0.19" diameter). the frame made of .25x.375" aluminum flat bar. on closer look it might only need to support 2-4 ft-lbs

i think what might be happening is that the clamping force needed to keep the assembly from colapsing back down due to gravity is so high that the aluminum is getting ground away and the joints are loosening, although the fasteners are not.

 

[MintJulep]

If I understand what you are trying to do, the proper way is to use a tube as your pivot axle. Build up over the tube washers of appropriate friction (including between your members) and a Belleville or wave spring to maintain constant load. Clamp the whole thing down with a rivet of your choice.