F-5 output device failure mode

[frags]

Originally Posted by Russellc

Is there a prefered spring rate for this application?

Now you will have sleepless nights thinking if your washers provide proper pressure on FETs.

PS. I always use spring (split) washers. No problem so far.

[Blues]

Originally Posted by Nelson Pass

That's the best. You can buy them at various spring values.

Is there a fender washer (smaller hole) version of these?

[EUVL]

IMHO, a Belleville washer (or conical waher) first appears to be the ideal solution, in that it exerts pressure uniformly around the perimeter, which is not the case for the humble spring washer. However, if you would tighten a screw with a conical washer till flush (totally flattened), and then release it again, you would find that the conical washer would have been deformed and lost a sigificant proportion of its conicity (and hence the ability to generate preload force).

Or in simple English, a conical washer should never be tightened till flush, but only be tightened using some form or torque control, so that it is given the right preload within its designed working range.

Disc Springs to DIN 2093
(Note stress level vs deflection)

Heavy Duty Bolting Washers


Patrick

[AndrewT]

Just to confirm the earlier comments on the difference between "shakeproof" and "constant load" washers.

The split washer, the serrated washer and similar are to prevent an improperly tightened nut/bolt/set screw from vibrating off. Similar to self locking nuts.

The "Belleville" washer is one type of constant load type fixing. It is designed as an elastic "spring" between the underside of the nut/head and the device to be clamped.
A stack of Bellevilles can be used to arrive at different spring ratings.
Two Bellevilles one inside the other has roughly double the spring rate (force change as distance changes, eg. due to temperature exapansion) and double the holding power (twice the force).
Two Bellevilles facing in opposite directions have half the spring rate and the same holding power. Combinations of opposite and same direction series Bellevilles can achieve many different loads and stiffnesses.

[tinitus]

I expect the split washer will need too much pressure to have an evenly load, and not good

[AndrewT]

the split washer needs very little force to squeeze to flat.
As the fixing slackens off the two cut edges expand out and dig into the surfaces to prevent further unscrewing. By this stage the fixing is "loose" with very little clamping load. The split washer has done it's job. It stops the joint falling apart and the nut/bolt falling out.

[CanAm Man]

All,

Just my two cents. Consider using a thread-locking compound, such as Locktite 243. We routinely build high-performance road racing engines that rely upon accurate and reliable fasteners, to endure through many heat cycles, and under the worst condidtions. We would not assemble a $50,000 engine without the use of Locktite on critical fasteners.

I'll be using it on my F5 MOSFET machine screws. If you use it, just be careful--a little goes a long way, and avoid getting it on the heatsink base or the heatconducting substrate of the MOSFET. I have never considered the "thermal impedance" of Locktite, but I'd assumine it's not a good conductor of heat.

[Nelson Pass]

Originally Posted by Blues

Is there a fender washer (smaller hole) version of these?

McMaster Carr

[bear]

The split one is commonly called a "split ring lockwasher"... fwiw. At least it is around this part of the NE USA...

There are also internal and external tooth lockwashers...

I've never had a problem with screws holding output devices getting loose when properly torqued and using a "tooth" type lockwasher on the nut side. I also use a small washer to spread the down force on metal tab type transistors, not on older TO-3 type metal packages.

Locking compound would not hurt... I guess... the Belleville washers are nice... but I personally have never needed to use them...

_-_-bear

PS. if only a 4-40 size was the available through hole (a plastic pak that can't be drilled) I'd
add a bar to hold the entire face of the plastic pak device as well... talking F5 sized power devices,
not <5watt plastic paks...