[R-390] replacement R390A gear clamp source

[Jim Whartenby]

My feeling is that there is a tendency to over tighten screws.  The result of over tightening is that either the screw or the split clamp will fail.  It must be remembered that the following paragraphs were written for NEW equipment assembly.  But we are working on the very same equipment some 50 years later.  So all of the hardware has been stretched, compressed and stressed so the torque needed to make a tight mechanical connection is much less then what was needed for initial assembly.  Perhaps if the screw threads are lightly lubricated much less torque will be needed to achieve a snug fit that will not slip?

>From the Collins Quality manual:

3.1.6    Tightness 
All threaded fastenings must be tight. Torque wrenches or torque screwdriversoften aid in assuring proper tightness of threaded parts and should be used wherescrew torque is specified. An adequate concept of "tightness" cannot be basedon torque alone, however. It must be considered that torque is just the meansby which tension is developed in the shank of the tightened screw. This tensionmust compress the parts being fastened sufficiently to withstand separationforces such as vibration or shock that may occur during the useful life of the equipment. Over-tightening may result in broken screws, either at initialassembly or at some later date.Many factors affect the optimum torque required for driving small machine screwsto a point where the proper tension is reached but not exceeded. These factorsinclude friction at the bearing surfaces, friction of the mating threads, lengthof thread engagement, presence or absence of lubrication, type of lock washer orlock nut, and hardness and surface finish of the mating parts.Most materials commonly fastened by machine screws (including metal) are compressible, and all screws are elastic within narrow limits. A few hours or daysafter initial tightening, a properly tightened screw is sometimes pronouncedslightly loose because of metal fatigue in the stressed screw and/or creep ofthe compressed material. Repeated tightening in such instances can be moredetrimental than beneficial. Reliable locking devices will compensate adequatelyfor this apparent relaxation.Because of the many variables affecting hand-tightening, it must be stated thatno simple chart of tightening torque or easily applied "rule of thumb" has yetbeen devised that will replace a good assembly operator's judgment and experience. However, when fasteners loosen and cause equipment failure, they usuallydo so, not because of failure of the locking device, but because of insufficientinitial tightening. 

4.2.6    Clamping Adjustable Gears
On gears utilizing split hubs for the purpose of adjustment in a gear train, thereare two common methods employed to tighten them to shafts: (1) a collar with twoor more set screws, or (2) a split clamp with one screw. In all cases where anappreciable amount of torque is involved, method (2) should be employed. Thescrew through the clamp must be lubricated to prevent a false indication of tightness. It is preferable to specify a tightening torque on all such clamps. Aftertightening, the "lips" of the clamp must not be touching. The set screws orclamp screws should be accessible at the point of normal adjustment. Wheremethod (1) is employed, care should be used that the screws do not engage theslot of the split hub.

This said, I have a limited number of split clamps from Collins equipment that I have scrapped over the years.  Most are from KWT-6 mechanical assemblies but a few are from R-390A receivers.  You can't save everything so I disassemble as much hardware as I can and scrap the rest.  
 Regards,
Jim

Logic is a systematic method of coming to the wrong conclusion with confidence.  Murphy 

    On Sunday, July 19, 2026 at 06:51:24 PM CDT, Paul Roche via R-390 <r-390 at mailman.qth.net> wrote:   

 This must have been discussed 1000 times but I can’t find it. Does McMaster Carr carry replacement clamps that would work for the 3 or 4 sizes needed? I have a broken 0.312” (5/16”) clamp on my VFO. This is my third broken clamp in this unit; I am expecting more. 

Paul
Sent from my iPhone
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