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Mildly interesting - post mortem of failed Westach ammeter

Last annual, I replaced the ammeter due to the old one going intermittent then failing completely. I’ve been meaning since then to post-mortem the dead one (I was sure it would be some kind of loose connection inside). Here’s what’s inside one of these ammeters, which are a pretty common type in the GA fleet…

Back case:

The guts:

What went wrong: the terminals are kind of copper ‘rivets’ and are attached to the ring terminals on the end of the as a press fit (presumably attached with some kind of tool that flares the end over the ring terminal). Over the years it worked loose.

Andreas IOM

Thx for posting, Alioth! Very interesting failure mode
The stress on that terminal seems very minor…is there some movement to be expected on t his rivet either inside or outside?
Otherwise it was an evident quality failure in pressing the rivet/eyelet and the loose attachment was bound to fail sooner or later…surely when pulling on the yellow wire it stays firmly attached, right?

Antonio
LESB, Spain

I have passed to my engineers, who have been trying to diagnose a failed ammeter for three years!

EGKB Biggin Hill

I find it much more than “mildly” interesting which is a bit sad! There should be another part, perhaps on the back of the Bakelite which should be a thick wire/ shunt.

Perhaps the shunt is external – I am interested as if the copper terminals (which have been swaged on) have worked loose, perhaps it is due to thermal expansion/contraction rather than vibration?

United Kingdom

Yes, really sad. That instrument was barely over 50 years old, atrocious that it failed with less than a century on the clock.

Biggin Hill

Timothy wrote:

I have passed to my engineers, who have been trying to diagnose a failed ammeter for three years!

Are you sure they are electrical engineers? I would not be surprised if a mechanical engineer struggle that much

Cobalt wrote:

atrocious that it failed with less than a century on the clock.

On those timescales cosmic radiations and mutations will start to matter for avionics !

Last Edited by Ibra at 16 Mar 22:24
Paris/Essex, France/UK, United Kingdom

With that build quality it is amazing it lasted 5 years

It is interesting that they apparently used a press tool to press out the entire “PCB” including all the holes, and then put the rivets where they wanted connections, and that this was seen as cheaper than having a normal PCB. It is a pretty extreme level of cost reduction, but it enables you to make many different similar-looking instruments using that same piece of paxolin.

Administrator
Shoreham EGKA, United Kingdom

I’m not sure it’s 50 years old — Westach still make this exact ammeter today (and it sells for over US $100). The shunt on this one is external (I think there are versions of this with an internal shunt too).

There’s no strain on the wires going to the connector pin on the back. There should be minimal thermal effects, given the external shunt the amount of power flowing through this is minuscule.

Andreas IOM

It’s not that old – the extra four digits in the postal code printed at the end of the Sonoma, CA address were introduced in 1983.

This seems to me typical construction for automotive, commercial and light aircraft stuff. It’s strange that it failed and the only thing that comes to mind is that it was badly swaged and loose when new, then somebody rotated the lug on the exterior of the case trying to tighten it somehow and it popped loose. Not sure that makes sense but anyway small machine brass screws are cheap and it could easily be repaired for use as a spare… whether strictly legal or not.

Last Edited by Silvaire at 17 Mar 16:45
9 Posts
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