TO-220 'lock' washer

I found this, but not much info on the washer.

GH

Wavy washers are nice.

I found a lot of stuff on the web, but mostly about getting heat out. (which I don't care about.) I think maybe stack a few bellevilles in opposition to get more defection and less force.

Do they make wavy washers small enough for to-220? (#4 or #6 screw)

GH

I follow the advice in a Motorola/ON Semi SCR/Triac manual which is to place a flat washer between the TO-220 tab and the belleville. Also be sure to use a torque driver set to the recommended torque (I forget right now which figure). Belleville is nice because it is kind of constant force.

piglet

I have seen metric stainless wavy in all sizes from M2 (2mm dia) upwards.

piglet

The first place to look for any kind of hardware what-if question is

The circular mark has cut right thru the plating into the tab base metal

- I think that shows the system was way over-torqued.

piglet

mark

h I

And said torque would twist the tab leading to the case being under stress, if the leads were soldering in previously to the torquing down, and eventually separating as it did.

A tiny dab of Green Locktight would probably be enough for the 4-40 nut and washer if you are mostly looking for securing with a minimum of torque...

John

Which is why you have to tighten a Belleville to the specified _deflection_ rather than the specified _torque_. Otherwise you're liable to bottom it out and make this sort of problem.

Cheers

Phil Hobbs

The scraping of tin plate indicates they were installed bell-down, but not necessarily that force was excessive. More important, would be a torque scewdriver setting so as not to 'max out' the spring and keep going to higher force (hard edges made a scrape, cannot tell force from that).

Usually, one puts a spacer over the hole, and attaches through the spacer, so that the diameter of the washer doesn't impinge on the epoxy. Mechanical lateral force on a Belleville washer is HUGE, can easily crack things. The spacer, on the other hand (like a washer) spreads the steel screw pressure over the softer copper of the TO-220 tab. Loctite makes something that will retain the screw (but nickel plate screw into galvanized or aluminum doesn't play nice with the iron-catalyzed variants).

Thanks piglet, I've always used a flat washer on the 'bottom'. We'll probably add one.

George H.

Right, I think that's how I found Seastrom. (McMaster-Carr has a nice search engine... their index is a thing of beauty too!)

George H.

Yup, thanks everyone for the advice.

George H.

As a temperature sensor only, the issues with mounting of power-dissipating TO220's go away. Any lockwasher should work, if applied as designed.

Sensors may not employ the same solid-filled epoxy package structure as power semiconductors - issues with mechanical damage are different, and the requirements for environmental protection come from a completely different book.

RL

... Lock washers are useless. How many are in your car?

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

void _-void-_ in the obvious place