Flat Washers in C/Bore Hole - Aluminum 6061-T6 Material

[Vah1D]

Hi everyone,

We use a lot of aluminum in our projects. Our general practice is to use steel flat washers under the head (we mostly use socket head cap screws) to avoid leaving indentation marks, or damage the surface during tightening and that seems to be working well.

However, we don't use them when we have c/bore holes, and sometimes we see the marking left by the screw on the disassembled parts, although it's not frequently.

For standard size c/bore hole, I'm forced to use small OD (very close to bolt head diameter), so I'm not sure that will solve the issue (maybe only helps with not leaving marks during turning) since the bearing surface remains the same with or without using the washers. Has anyone had experience using washers in c/bore holes?

Just to clarify, I try not to avoid c/boring for no reason, but sometimes it's necessary for different reasons. So let's assume re-designing to through-hole is not an option.



Thanks!
Vah1D

 

[BrianPetersen]

My main experience has been with motorcycle engines. On big fasteners such as cylinder head bolts/nuts (depending on whether the engine uses cylinder head bolts or studs), there are always washers underneath the bolt heads, they're always slightly bigger than the diameter of the bolt head, and the counterbored hole (if that's the design used) is always big enough to have clearance around the washer. Disassembly sometimes involves fishing them out with a magnet.

 

[rb1957]

surprised you use steel washers on aluminium parts ... I'd've thought that the steel would damage the Al surface as much as a fastener head.

I'd use Al, or phosphor-bronze, washers.

I'd use a small washer in a counterbore.

You may use teflon washer under the metal washer, to reduce friction.

another day in paradise, or is paradise one day closer ?

 

[mfgenggear]

aluminum threads strip out very easily. I don't recall how many major installation fixtures I have repaired by installing steel inserts. problem solved.

 

[rb1957]

it should be relatively easy to calculate the torque that's cause AL thread to strip.

another day in paradise, or is paradise one day closer ?

 

[BrianE22]

Can you make bigger counterbores? Are the washers at least a bit larger diameter than the head diameter? If so then a thicker washer may spread the load out more per the "cone" effect (my term - most of the bolt load spreads out within, I think, a 30 deg. included angle taper).

Getting the head of the bolt to spin relative to the washer (grease?) rather than the washer spinning relative to the aluminum bore would help a bit.

 

[mfgenggear]

rb1957

it would be , the issue is dissimilar material, the threads seize then on disassembly strip out. on vehicles with aluminum heads, & spark plugs, I add anti seize and use hand tight, from years of turning a wrench, I can tell when the spark plugs are tight enough. then check it with a in/lb torque wrench. I don't get over zealous with the socket and ratchet.

 

[TugboatEng]

Galling is an issue with aluminum so predicting the failure torque is challenging.

As for the washer, even if the diameter is small and it doesn't increase bearing area, the steel on steel pair may favor the sliding motion over the steel on aluminum pair which would reduce the wear on the aluminum part.

Why is this a concern? Does this fastener see frequent assembly and disassembly?

 

[rb1957]

few things better for tightening a bolt, than an experienced hand at the other end of the wrench !

I pictured striping as shear the threads as the screw turns without travelling along it's axis ('cause the head is seated).
Maybe too simplistic ...

another day in paradise, or is paradise one day closer ?