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Dewhurst reversing switches

An abomination!

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Kiwi Bloke17/09/2019 12:00:14
250 forum posts
1 photos

There's recently been some discussion about Dewhurst reversing switches in MEW and this forum. In MEW No. 283, Glyn Davies advocates the only sensible solution - throw the Dewhurst away! There's no need for these expensive abominations these days.

We all owe it to ourselves and our families to take basic precautions to preserve our health and well-being. Serious machines really should be controlled by a DOL starter / no-volt release. I don't believe the Dewhurst switch was designed to switch machine motors on or off - the make and break action is too slow and the contacts are rather feeble. It is fine as a switch to select forwards or reverse*, but not to start or stop the machine. Unfortunately, this seems to be its most popular application, and so the poor thing eventually burns out...

* Well, clearly it wasn't in Glyn Davies' case.

not done it yet17/09/2019 12:43:56
3341 forum posts
11 photos

Nothing on this forum (according to a search for ‘dewhurst&rsquo this year, apart from the enquiry about the missing refurbishment article (in MEW 283 instead of 282).

As I see it, they have worked quite well as an isolator and direction of rotation selector for a small motor. No reason to assume they have stop/start capabilities - which is where the problem lies. Education needed, not necessarily banning the use of these devices. But, sadly, Common Sense died years ago and many either don’t know or don’t care.

I’ve never looked up the current carrying or switching capability, but would avoid exceeding that if I were using one. As for stop/start of an inductive circuit, it is a non-starter (apologies for the pun!)

My previous lathe had a different switch, but the same idea. Sensibly, there was a starter to be used for that. One should never use an isolator to switch current, in my book, unless in an emergency situation.

But, there again, I worked on a plant where there were in excess of 150 drives (probably over three times that number, over the whole works) - each with a starter and an isolator. In the English language (electrical parlance) the two are not synonymous. Simple as that. Comprendez or non-comprendez is the problem. Elf,n,safety likely ban their use in industry and insist on interlocks to prevent similar items causing problems.

I suspect a fair number of failures are induced by switching direct from one rotation to the other without allowing the drive to stop, even if the normal starting current does not exceed the specification.

Michael Gilligan17/09/2019 14:36:07
avatar
13975 forum posts
605 photos
Posted by not done it yet on 17/09/2019 12:43:56:

[ ... ]

apart from the enquiry about the missing refurbishment article (in MEW 283 instead of 282).

.

My opinion ... as per [as it stands] my closing remark on that thread:

**LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=142855

MichaelG.

Lambton17/09/2019 16:28:48
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682 forum posts
2 photos

A few years ago I wrote, and had published, in Engineering In Miniature (July 2014 Vol. 36 No1) an article entitled Protecting a Dewhurst Switch, the Lathe and Yourself ".

The article covers the limitations of a Dewhurst switch and how to combine it with a DOL starter providing No-Volt release, and having normally closed contacts in the start hold-on circuit, how any number of safety stops and interlocks. can be easily provided.

Crucially the Dewhurst is then used only to select motor direction of rotation and nothing else.

I wrote this article as many hundreds,, or maybe thousands, of Dewhurst switches remain in use on older Myford lathes and other small machines. and can be made to work safety and effectively.

I am not sure if EIM provide general access to published articles. .It is also fully appreciated that modern control gear that eliminates Dewhurst type switches is available.

Eric

Andrew Tinsley17/09/2019 16:42:11
919 forum posts

I have several Dewhurst switches in my workshop, some on very old machinery, so they have had some considerable use. Never had a problem with them in all my years of using them.

But then I suppose they are pretty useless.

Andrew.

SillyOldDuffer17/09/2019 16:46:15
4696 forum posts
1010 photos
Posted by not done it yet on 17/09/2019 12:43:56:

...

As I see it, they have worked quite well as an isolator and direction of rotation selector for a small motor. No reason to assume they have stop/start capabilities - which is where the problem lies. Education needed, not necessarily banning the use of these devices. But, sadly, Common Sense died years ago and many either don’t know or don’t care.

...

I suggest there is no such thing as common sense! Everything we know is learned from life experience. Give Mr Common Sense a new problem and watch him struggle when lack of appropriate experience lets him down with a bang.

Common sense can't tell anyone if a switch has stop/start capability or not. You either have to read the instructions or look inside and evaluate the construction using prior electrical knowledge. Common sense doesn't provide any insight into inductive loads or reveal what difference that makes to switch design.

While there's no reason to suppose a Dewhurst has stop/start capability, there's no reason for a beginner to assume it doesn't! Common sense is just a guess. Based on size and application, it's surely not unreasonable to assume a Dewhurst ought to be capable of switching loads? I know it's not because Myford owners have complained about them since 1950. Truth is the Dewhurst is a chunky old-fashioned design dating from before the era of No-Volt Releases and it was never intended for hot switching motors as might be expected on a lathe. It looks like a quality switch and isn't. Dewhursts are only fine if you understand their limitations.

One thing I do like about Dewhursts is they look right on a Myford. Garish modern switches tend to clash horribly with the styling of older equipment.

Dave

Howard Lewis17/09/2019 19:52:46
2327 forum posts
2 photos

Common Sense is not that common, unfortunately!

There have probably been very few cases of Myford owners, with Dewhurst switches, being electrocuted or incinerated.

And, over the years, a large number of Myfords must have been fitted with Dewhurst switches. They were virtually a standard fitting. The one on my ML7 never caused me any problems.

But the switch was like the machine; by modern standards old technology, unsuited to our modern world where before picking up a sharp pencil a written risk assessment.seems to be needed.

Howard

Jon Lawes17/09/2019 21:07:48
avatar
324 forum posts

I have installed another switch inline with my dewhurst and only use it for reversing. I got bored of cleaning nadgered contacts.

Consider that rather than lacking in common sense it might be that beginners such as myself has less mentoring available. Although its fairly obvious you should take the chuck key out before starting the lathe I suspect many experienced people (and myself) have done it anyway by mistake. The Dewhurst switch looks like it should do the job but doesn't; common sense didn't help me with that. I just assumed it was fit for purpose (wrongly).

Common Sense isn't dead. But you get knowledge through being taught or learning yourself. One of the guys I used to work with refused to have an apprentice as he said he would probably take his job one day. Blame people like that if there is a decline in young skills.

Meunier18/09/2019 10:15:38
245 forum posts
1 photos
Posted by Howard Lewis on 17/09/2019 19:52:46:

..../., before picking up a sharp pencil a written risk assessment.seems to be needed.

Howard.../

Might that be putting the horse after the cart, Howard ?
DaveD

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