I am aware of how ‘differential screw’ adjusters usually work [screw threads with different pitches, working in tandem] … But these have me stumped:

**LINK**

https://www.ebay.co.uk/itm/254633999168

Could some kind soul please post a rough sketch to explain how left and right hand threads of the same pitch can provide differential adjustment ?

… My brain hurts !

MichaelG.

I don't think they are the same pitch.

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I do, John … that’s why it’s puzzling me

M8x1mm

Left-hand one end, Right-hand t’other

MichaelG.

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Credit Tony4Cats

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MichaelG.

Edited By Michael Gilligan on 29/10/2022 22:10:50

How does that make them differential?

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That was my question ^^^

They are listed as such

MichaelG.

They are not really what I would call a "differential screw adjuster". As you say, they usually have threads of different pitches and are the same hand to make fine adjustments.

These are just like the threads in the middle of a pair of spring bow compasses, or track rods on a car. As you wind the screw the "nuts' on each end are either pulled together or pushed apart. An adjusting screw, but not differential. Perhaps the description sounds more technical for an advertisement than simply calling it a left and right handed adjusting screw!

Neil

In the absence of any explanation … I think you must be right, Neil

I asked the question because I was beginning to doubt myself

… Looks like I might be O.K. after all

MichaelG.

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Trouble is … He also lists these: https://www.ebay.co.uk/itm/313125445600

… which make far more sense.

Edited By Michael Gilligan on 29/10/2022 22:49:36

Get over it. He just did a cut and paste job on the description without thinking much about it. If you look at his sales count he is a garage trader shovelling out the stuff by the wagon load so isn't going to put a measurable amount of QA checking on his sales pitch.

At sea we called them bottle screws but turnbuckle or tensioner screw is probbly a better description

I don't know about their use in engineering tooling, but, the ones in your first picture are used in spindle moulder tooling, (wood machining), along with an olive, (cone shaped nut), to hold the wedge, which locks the knives into the block.

It is not very obvious how they are used from the links below, I can takeyou a picture on Monday if you want.

Screw

Olive

Mark.

Good morning.

We use them at work to hold a carbide insert into the toolpost of a scarfing machine on a continuous ERW tube mill.

I recently bought some spare ones for the machine and Sandvik do indeed list them under the name 'differential screw'

I assume that this is the literal translation to English from the makers mother tongue.

Cheers

Dave

Well, Gentlemen :

… aside from Bazyle’s rather stroppy “Get over it.” … I’m grateful for the input.

MichaelG.

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P.S. “Garage Trader” he may be … but he lists some very useful stuff

In some sense, they _are_ differential because the rate at which the screw advances (2mm) is different to either of the pitches cut on each end.

If you have assimilated a definition of a differential screw _requires_ it to have different pitches at each end, clearly they do not fall within that.

The eBay listing is confusing as it remarks on the fine adjustment ability of a differential screw, a property which is only true if you adopt the latter definition.

If the diameter is fixed at 8mm, and the application needs 2mm advance, the other option is a 2-start thread, but maybe that is more expensive to produce than the Pushmi-Pullyu.

Thinking about it some more, if we wanted to write a generalised mathematical expression for a differential screw, we would need two inputs for each end of it: the pitch of the thread and the hand of the thread.

If we agree a convention that the pitch of a RH thread is a positive number, we can represent a LH thread in our expression with a negative number.

The fine adjustment mechanism has the same hand at each end and different pitches. We subtract the two pitches and take the absolute value of the result to calculate the differential movement.

That formula works for a length of threaded rod: same hand and same pitch - do the subtraction and you arrive at a differential movement of zero as expected.

For the eBay one, the hand is different but the pitch is the same. Do the same subtraction, but remembering our LH thread is represented as negative, and we arrive at a differential movement of 2mm in this case.

Let us say we have a 1mm RH pitch one end and a 1.25mm pitch LH at the other end. The differential in that case would be 2.25mm. If you happened to need this movement for something, it is hard to buy a tap or die as it is a non-standard pitch. Because of this, it might not feature in the gearbox of a screwcutting lathe.

Edited By DC31k on 30/10/2022 08:28:11

Hi Michael,

From Wikipedia:- (in case of a single lead equal to the thread pitch), and possibly opposite handedness, on which two nuts move. As the spindle rotates, the space between the nuts changes based on the difference between the threads.

This is not the full description, but the word "possibly" may mean not everybody agrees it's differential.

If they are not left and right hand threads I don't see how it could be differential.

My head hurts all the time now!

Cheers Michael.

The left/right handed combination screws are sometimes called compound screws and other times clamping screws too.

The Free Dictionary defines a Compound Screw thusly:

So it seems both differential and right-and-left screws are types of compound screw. But on the shop floor the blokes using the screws to retain the carbide inserts probably don't give a toss and just refer to them generally as differential screws and get on with the job. Well, the thread on each end is different from the other, in handedness. That's differential enough for practical purposes.

Good to see this discussion developing

I will just mention Norris … who deserves the credit for my first introduction [decades ago] to differential screws:

**LINK**

https://www.leevalley.com/en-us/discover/revisiting-the-past-the-lee-valley-antique-tool-collection/featured-patent-norris-plane

MichaelG.

If these screwed studs have the same pitch, and the same hand at each end, rotating the stud, between two fixed members, will merely move the stud between the fixed members, without changing any dimension

If the stud is stationary, and one end member is rotated there will be a a change of overall dimension.i

This what happens with a cylinder head stud on an engine, although it not unknown for threads of different pitches to be used, such as BSW into the cylinder block and BSF for the outer end,.

If the end threads are of different pitch, but the same hand, there will be a fine adjustment, because the advance at one end will differ from that at the other. The coarse fixed end reduces the risk of stripping the fixed thread, while the finer outer thread allows a clamping load to be applied with greater accuracy.

If the threads are right hand at one end and left hand at the other, and the stud is rotated, the device will act like a turnbuckle, either drawing the fixed members together, or pushing them apart..

In the case of a turnbuckle, the end studs remain stationary and the central member rotates to produce the adjustment.

To produce a differential adjustment, the threads have to be different, either in pitch or hand, (As shown in the picture ).

Howard

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[my emboldening] … Which is very clearly the case in the example under discussion

It was the leap from ‘turnbuckle’ to ‘differential screw’ which troubled me enough to ask the question.

I shall be very interested to see the detail that Mark has kindly offered to post tomorrow

… largely because the ‘Olive’ that he linked only served to confuse me further.

MichaelG.

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https://www.whitehill-tools.com/spares-accessories/screws-olives/470S00070/

Edited By Michael Gilligan on 30/10/2022 10:21:31