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#2091987 - 05/30/13 08:37 AM Threads on tuning heads
Mark R. Offline
2000 Post Club Member

Registered: 07/31/09
Posts: 2265
Loc: Pretoria, South Africa
My Schaff extension lever is the only lever I own, so I don't know what system other manufacturers use. But I've been wondering why the Schaff/Hale system uses a conical thread for fitting the head to the shaft (to my knowledge, 1/8 - 27 NPT) but a cylindrical, flanged one for fitting tips to the head (25/64 - 30).

Of the two, the head-to-tip fitting seems much more secure and "positive" to me, because it relies on friction between the tip and the flange/shoulder on the head, while the NPT thread relies on friction in the thread itself.

When fitting a head, there is no positive stop-point, and I always get horror visions (irrational?) of splitting the head while tightening it on the shaft.

The masters must have had their reasons for this design... I do realise that the shaft-to-head fitting transfers sideways force through the threads, while the tip-to-head thread transfers torque. Still, I do wonder what the rationale for the conical thread on the shaft-to-head fitting is. Any ideas/comments?
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#2092154 - 05/30/13 01:09 PM Re: Threads on tuning heads [Re: Mark R.]
Emmery Offline
2000 Post Club Member

Registered: 04/02/08
Posts: 2481
Loc: Niagara Region, On. Canada
I don't have that particular design on mine, but as a machinist for may years I'll comment on the NPT vs straight theads.

NPT is typically used for pressure fittings where gas or liquid is to be retained and not leak. It is also used on fittings where no shoulder/lip or bearing surface is available to secure the fitting, it relies on a gradual interferance fit. NPT is also used in combination with threading depth gages to secure a fitting at a specific location along the threads or with a tight fitting tolerance on how much the fitting can protrude. Lastly, NPT threading is sometimes done on fittings that are meant to be permanent or semi permanent while additionally connected fittings with regular threads are intended to be regularly removed. An NPT fitting can be deliberately torqued on so tight (dry threads) that the threads gall with each other and its like having Loctite red on it. It can eliminate the need of holding it in place with a wrench when undoing the other looser fitting. To this end, on all other applications where removal is required, NPT threads are always lubricated with paste, oil or teflon tape...these prevent the threads from galling and seizing. Just my 2cents FWIW.


Edited by Emmery (05/30/13 01:18 PM)
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#2092667 - 05/31/13 05:49 AM Re: Threads on tuning heads [Re: Emmery]
Mark R. Offline
2000 Post Club Member

Registered: 07/31/09
Posts: 2265
Loc: Pretoria, South Africa
2 very valuable cents to me!

Originally Posted By: Emmery
I don't have that particular design on mine


Sorry, perhaps I didn't describe it very well. I'm not sure what lever you have, but I was simply referring to the threads on any standard Schaff head:


The female thread that accepts the lever's shaft is conical and devoid of a bearing surface, while the male one that takes the tuning tip is cylindrical, with a bearing surface. I was wondering about the use of two such different systems on one head. I think the answer lies in the motion that each one transfers: respectively lateral (shear?) force vs. torque.

Many thanks for your comments on NPT threads. The term, "gradual interference fit" makes a lot of sense to me. The shaft is fitted/locked into the head over several thread pitches. Therefore the lateral force of the tuning lever is distributed along the entire length of the interlocking threads - not only one bearing surface.

The tip thread, on the other hand, has to transfer torque. And this is apparently best achieved through the bearing surface.

By the way, the abovementioned thread measurements were taken from an article available through Renner, "Tuning Lever Design & Maintenance" by Keith Bowman, RPT.

Thanks, Emmery!
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Autodidact interested in piano technology.
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1922 49" Zimmermann, project piano.
1970 44" Ibach, daily music maker.

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#2092702 - 05/31/13 08:24 AM Re: Threads on tuning heads [Re: Mark R.]
Emmery Offline
2000 Post Club Member

Registered: 04/02/08
Posts: 2481
Loc: Niagara Region, On. Canada
Mark, I beleive the reason for the taper thread on the head to shaft joint is partially for the fact that this is the only solution for a solid locking thread joint that has no perpendicular shoulder for a bearing surface. (without resorting to adhesives)

In regards to transferring lateral shear forces as you mention you are correct. Normal straight threads have clearance in their thread pitch tolerance (class 2A)up to 4-5 thousandths of an inch for this size diameter. When tightened on a flat bearing surface the concentricity between the fittings shift off center in the direction of pull on the wrench tightening it. They are not fully self centering like tapered threads and don't distribute the holding force equally on the full perephery of the thread. It is extremely expensive/inpractical to tighten the thread pitch tolerances to eliminate this.

I do this work when I blue print a rifle barrel fit to an action. It requires taking successive small thread cuts with single point tool, stopping, and then trying to fit the parts. This is done back in forth until the parts snugly fit with very little or no play. Extremely time consuming. On mass production, this issue is eliminated by using self centering taper threads.

Also, conventional straight threads with a shoulder require it to be perfectly perpendicular to the thread axis to prevent shifting similar to what I mentioned above. The only way to do this practically is from the same set up and machine used in the threading operation, and this option is not always there in manufacturing on screw machines.


Edited by Emmery (05/31/13 08:26 AM)
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