Wiper Drive Pimary Link, what does the "eccentric" do ?

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rfs1957
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#1 Wiper Drive Pimary Link, what does the "eccentric" do ?

Post by rfs1957 » Sat Nov 30, 2019 11:02 pm

I hunted for 20 minutes through "Wiper" topics using the search, honestly, but almost everything seems electrical or motor-based, and not much mechanical.

Could anyone shed any light on this please ?

I have the guts of the windscreen-wipers in bits, motor works fine, so does all the mechanism as I overhauled most of it a few years ago, BUT I'd never had the primary linkage off, the first rod that is driven by the motor crank, until now.

Image

Could anyone tell me quite what has been botched here, and suggest why ?

Image

Is there an easy way (replacement part from another car ? available new ?) to get rid of the slop and wear that's present both in the centre bronze bush (which would be a bit of bitch to make/replace as the core is hard to get hold of) and in the interface between the eccentric core and the operating arm ?

Image

Are these even E-Type parts ?!

It would make everything a lot quieter if I could improve this.

Of course, it never rains here, and I wouldn't go out in the rain anyway, but, when I occasionally hit the switch by mistake, it doesn't sound very cool with all that click-clacking.
Last edited by rfs1957 on Sun Dec 01, 2019 12:23 pm, edited 1 time in total.
Rory
3.8 OTS Cream 877393 Built May 28th 1962
1978 Mini Van
(plus bevel and belt single-cylinder Ducatis)

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abowie
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#2 Re: W-Wiper Drive Eccentric ? Linkage onto motor ?

Post by abowie » Sun Dec 01, 2019 1:56 am

Can't help with a changeover period replacement but I think SNGB sell a repro.
Andrew.
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#3 Re: Wiper Drive Pimary Link, what does the "eccentric" do ?

Post by rfs1957 » Sun Dec 01, 2019 1:00 pm

I'm inclined to think these are original parts, and that the brazed reinforcement is just a repair rather than an adaptation ?

This picture

Image

from the Factory Fit string suggests plausibly similar parts, complete with a cover 54712745 that would have protected the "eccentric" and its spring, and that I don't have ?

Image

The "Link.primary" is 54711297, and the Link.rotary 54710596, all referred to in the J30 parts manual.

Has anyone ever overhauled / repaired /rebushed one of those Link.Primary, and do we know how many cars used this arrangement before it was replaced by something else ? I can't find any pictures of this mechanism, with the exception of Angus' shot when helping with a recent clip identification.

Furthermore, can anyone work out what this complexity was for, even ?!

Image

Image

When pushing on the primary link, the eccentric adopts - initially - the path of least resistance, and the link goes "short" with an eye-to-eye of 275mm, and then when the effort goes to pulling on the link, the eccentric rotates to the other extreme and the link goes "long" to 284mm.

Image

Image

I've not mocked this up to examine closely what the consequences are, but it's fiendishly clever and is giving me headaches.

Something tells me it's to give the wipers a certain "pause" at TDC and BDC, as each time the eccentric rotates there will be a brief asynchronicity between the motor rotation and the wiper motion, maybe so they were easier to park ?

Edit : I'm wrong in this interpretation ; the fact that there is the conical spring maintaining a certain load on the eccentric, in fact it always adopts the same position when the motor is driving in one direction, and then adopts the other position only when the motor reverses to stop.
Last edited by rfs1957 on Sun Dec 01, 2019 1:12 pm, edited 1 time in total.
Rory
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#4 Re: Wiper Drive Pimary Link, what does the "eccentric" do ?

Post by rfs1957 » Sun Dec 01, 2019 1:03 pm

Now I've changed the wording of my title, the clever Forum suggestor-mechanism has thrown up some similar posts, which I think I am about to find contain the answer to my questions ..............

http://www.xkedata.com/resources/read/?id=9

viewtopic.php?f=4&t=6509 - Kingzetts never got any follow up to this, however.

viewtopic.php?f=4&t=4600 - and this one ran dry too.
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Robbiee
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#5 Re: Wiper Drive Pimary Link, what does the "eccentric" do ?

Post by Robbiee » Sun Dec 01, 2019 4:37 pm

Hi
I bought a primary link from Richard smith
Robbie
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#6 Re: Wiper Drive Pimary Link, what does the "eccentric" do ?

Post by christopher storey » Sun Dec 01, 2019 7:47 pm

Rory _ I am not certain, but I think the eccentric is something to do with the parking mechanism, when the motor runs backwards - during the change from forard to astern the wipers do that extraordinary dance

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#7 Re: Wiper Drive Pimary Link, what does the "eccentric" do ?

Post by MarkRado » Mon Dec 02, 2019 4:39 am

:yeahthat:
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#8 Re: Wiper Drive Pimary Link, what does the "eccentric" do ?

Post by Duckham » Wed Dec 04, 2019 10:47 am

Rory - I scratched my head over the reason for that mechanism when converting to RHD.
As no one at the time could explain it, when I finally understood I did a post which I just found linked in the knowledge base with some photos http://www.xkedata.com/resources/read/?id=9 where I wrote:

If interested in WHY this works though, read on:

If you take the control rod /arm off the wiper motor you will find that it actually has an eccentric cam (i.e. the hole for the spindle is offset from the centre of the cam) built-in, with ‘stops’ that give it 2 possible positions.

When the motor turns clockwise the cam is forced against its’ stop in one direction, when it turns anti-clockwise the cam is forced against the other stop. Because it is eccentric, the effect is that the rod has a shorter overall length when the motor turns one way than the other.

The reason becomes clear when you realise that switching the wipers to OFF actually puts the motor into REVERSE. It is not until the limit switch gets to the end of its travel that the motor stops.

So this is the way it all works:

Wiper Blades are normally parked tight against the windscreen rubber surround so as to be out of the way.
Switch the wiper ON (slow or fast) and the motor turns one way, forcing the cam into a position which leaves the control rod at its ‘short’ length.
When the blades complete a sweep and come back down to the bottom of the screen this ‘short’ length means they stop well before the surround (to avoid hitting it) and continue the next sweep. The ‘finger’ of the limit switch never leaves the brass surface.
Now move the wiper switch to OFF and the blades will actually reverse their direction of sweep, back up to the vertical position. The cam is now thrown to its other stop, effectively lengthening the rod.
The extra length means that when the blades sweep towards the bottom of the screen this time they keep going until they touch the rubber surround. At this point the ‘finger’ runs off the edge of the brass plate and the motor stops.

Joe
63 OTS

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#9 Re: Wiper Drive Pimary Link, what does the "eccentric" do ?

Post by rfs1957 » Wed Dec 04, 2019 12:50 pm

Hi Joe, Yes I had discovered your illuminating explanation and posted it in #4 of this string, but the more links there are to it the better !

It's indeed a very clever bit of chicanery, whoever thought that up must have been an insomniac.

I seem to have spread this debate over two topics

viewtopic.php?f=7&t=15116

so if you want to clean up the mess, David, feel free to amalgamate.

From the parts list drawings, I'm tempted to speculate that this same eccentric idea may have been used on the cable-rack type of wipers too, as there appears to be a similar 3-rivet assembly visible here too ?

Image
Rory
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