8/11/2019 Differential Screw Fittings

1/1

Differential

WORKSHOP HINTS AND TIPS

screw fittings By GEOMETERCCORDING

Ato what may be

required in a tool or mecha-nism, a very large axial

force, or extremely fine adjustment,can be provided by a differentialscrew, using normal pitches ofthread of robust proportions andample strength. Even for appli-cations where the function is fineadjustment, these are importantfactors; for a thread of very fme

pitch has no great vertical heightto withstand wear , much lessprovide considerable axial thrust

without stripping.A differential screw is one having

two different t.p.i. or pitches; andit is by using the difference betweenthem that the force or tine adjustmentis obtained. For any particular pitch(which is obtained by dividing 1 bythe t.p.i.), a thread may be likened toa type of wedge. A large pitch(small number of t.p.i.) gives a steepwedge, and a small pitch a very slenderone.

Now, two steep wedges of slightlydifferent angle can be placed together

in opposition. The variation fromparallel is equal to that of a veryslender wedge. This is similar to adifferential screw. and it works asat A.

Making one forward turn with thescrew from the left side, the space Xis diminished by the difference betweenthe movement of nut Y with the finethread, and that of nut Z with thecoarse thread. If nut Y has 10 t.p.i.,the pitch is l/l0in.,or 4/40in.; andif nut Z has 8 t.p.i., the pitch is 1/8 in.or 5/40 in. The difference is l/40 in.,which is the pitch of a normal micro-meter screw.

Consequently, using these sub-stantial threads in a mechanism, itis possible to obtain the fine adjust-ment of a micrometer-or the forcethat attaches to l/40 in. pitch-withoutrisking the stripping that could occurwith a thread proportioned to l/40 in.pitch. Even threads of 4 t.p.i. and5 t.p.i., which are very strong, givethe equivalent of 1/20in. pitch; andat the other extreme, combinationsof threads of line pitch (large numbersof t.p.i.), relatively weak though theymay be, will provide an extremely

COARSE *THREAD \ SLEEVE

28 J ULY 1960 105

fine adjustment when this and notforce is the function required.

-The considerable force provided bya differential screw admits of assem-

bling parts with the minimum riskof their working loose; and often nolocking device is needed. A possibleuse in a small engine is as at B, forfixing the crankpin to the web of acantilever crankshaft. The crankpinis a sleeve (with flange to keep thebig-end of the connecting-rod on it)and is tapped with a fine thread,while. the crankweb is tapped with acoarser thread. The screw can be

made to length-or cut to lengthafter pulling the parts together.The force available in such a double-

pitch screw is particularly useful onoccasion in a puller for separatingtaper-fitted parts. It can be arrangedas at C. The normal pressure screwcan have its usual relatively fine threadand be fitted into a sleeve with acoarser thread to the body of the

puller. The pressure screw gives thesetting and initial tightening. Thenit is held while the sleeve is turned.This advances in the body and at thesame time draws the pressure screw

back; and the difference between thetwo movements is the actual advance.No hammering should be needed onthe pressure screw.

Parts of jigs and tools can be pulledfirmly together with a fitting as at D,obviating recesses or countersinksfor screws-the ends of the screwsgoing beneath the surface. Tightening

by ordinary screwdriver gives sutffcientforce with the fine functional pitch.Screws must be started with partsseparated an appropriate distance(right) for correct final assembly

(left).

Fine adjustment for the lens of amicroscope can be arranged as at E(top) with a double-pitch screw;while tine adjustment for a tool(bottom) is possible through a sleevewith different pitches inside and out.

For the latter fitting, a backingscrew is advisable to lock the toolwith backlash upwards. In adjustingthe sleeve, conditions as at F occurBacklash between sleeve and bar iscorrectly taken, but the tool is pulleddownwards, leaving backlash beneathits threads. So a backing screw will

prevent it from pushing down. q

MODEL ENGINEER