Problem with Aluminum chassis

Willie,

Suppose you could fabricate a support plate, that should help strengthen the weak spot. Is it the PT or OT?

David

 

What kinda amp is it? Depending on how thick the chassis is the plates could help straighten it out depending on thickness and how badly it's bent. Another precautionary type support would be to use wooden dowels as little " legs, think I saw something like that used on a Trainwreck cuz of the thin chassis.

I built a JTM last year and use a support ring for the PT and was thinking of using the wooden legs for the OT.

http://45watt.18watt.com/gallery/DBaileyClone/board_shot

regards

David

 

Hi,

I use large fender (not Fender) washers on all my tranny attachment points to help distribute the load a little further out. On anything thinner than .090" aluminum, you will want to do something like that.

 

Aluminum chassis area lot easier to machine than steel plus it's nonmagnetic so there are reasons a builder would prefer it but yes the thickness and I would think the alloy would matter how strong it will be. Actually I believe aluminum becomes stronger over a period of time or it can be tempered with heat.
I'm not a metallurgist so I can't give details but in the past I had some chassis made of 60/61 T5 aluminum .083 thick and I can stand on it without fear of it bending unless I jumped on it. Trainwreck amps have rather thin chassis but the trannies are mounted at one end of the chassis where it is the strongest, still I would be very careful.

db

 

Hi Graydon,

The fender washers sounds like a good idea, how big of a washer do you use?

David

 

David,

I use the largest fender washers I can comfortably fit on an 8-32 screw. They are about .75 dia or maybe 7/8" dia.

Willie,

I use .060" aluminum on my 18W chassis. It is strong enough for that application since the parts are relatively light. I use .125" aluminum on my 100W chassis and that is plenty strong enough to handle that amp's needs. It is also a much better design from a structural standpoint, with folded and welded corners and a fully enclosed design. The 18W is a pretty simple, chassis that isn't very strong. If you look at original JTM45s, they used aluminum chassis in .0625" and they are pretty wavy usually but they are hanging in there. I'd say with normal use, even a thin aluminum chassis will probably last a long while. You need to be careful though if you can.

Aluminum gets weaker with age. It has nothing to do with number of cycles although you can break it by bending it back and forth. Curiously though you can restore old aluminum to it's original strength (or pretty close to it) by heat treating it again. This property of aluminum separates it from steel which gets weaker with use, not age. If you bend steel back and forth a few times, it will crack and break. Aluminum will not do this (within normal use limitations.) Al is roughly 1/3 the weight of steel and about half the strength (which depends greatly on which alloys of each you wish to compare.)

I have some 7000 series aircraft aluminum that is incredibly hard and strong. It is as strong as many lower-med grades of steel. It is used on wing spars by McDonnel Douglas.