Effect of mass on stub acme thread?

[EngineerPhil]

I am experiencing pick up or galling on the mating faces of a threaded male and female stub acme part. Both are AISI 4145 and phosphated.

The thread is designated a 14.5" - 6- STUB ACME 2G thread. This is a nominal diam of 14.5" with 6 TPI. The female part is fixed level to the ground whilst the male part (weighing in at 292 lbs) is lowered vertically downwards. Just as the parts touch the male part is rotated anti-clockwise to ensure good engagement before being tightened clock-wise.

After about 3-4 turns the feeling of pick up is felt and when removed the mating shank faces have been scored.

Basically, my question is: Does the weight of the male part have an effect. Would I still get pick-up if could screa the parts together horizontally? Is there are general rule for weight of part and thread TPI?

Also, have I stated an incorrect TPI. would 2 or 4 be better? and should I have the threads hardened somehow?

I am abit stuck and really need help. We have spent alot of money on the rig and evenb with advise from the thread cutter, we still have problems.

Please help,

ta

Phil.

 

[desertfox]

Hi Engineer Phil

What are you using this rig for, I presume it is for power transmission and the shaft will be raising and lowering something during service?
From the information you have given it is not normally good practice to have the male and female threads that mate together made from the same material as similar materials
tend to have higher coefficients of friction than dissimilar
materials.In addition you do not appear to be lubricating the threads while assembling which would help your situation as the lubrication would reduce friction.
A rough calculation of bearing pressure on the threads due to the weight of the shaft is likely to be around 43lb/in^2,
if you were to assembly horizontally you may have problems aligning such a weighty shaft.
Could supply some more information about this rig ie:- what is it used for, length of shaft etc, is it possible to keep the shaft stationary and spin the female part if the female part weighs less than the shaft?

regards
desertfox

 

[EngineerPhil]

Desertfox,

Many thanks for your reply.

The rig is part of a pressure test rig and these two parts are similar to a reducing coupler for a 3/4" BSP water fitting. But its not!

That's why the materials are similar.

We have actually managed to mate the parts horizontally. This was difficult but the self weight did not seem to have any effect on the threads. We were supporting the male part fully using a pulley system and a fan belt. A rather big fan belt.

The male part is 12" and the thread is 4". As the female part weighs twice as much as the male part, spinning the female part isn't really possible.

I am just after a rule of thumb or some calcs so I can avoid using the wrong TPI for threads. The self weight is really important as it greatly effects the frictional force. The threads have never yielded and hold well under test pressure.

Any more thoughts?

phil.

 

[desertfox]

Hi EngineerPhil

I don't know any rule of thumb for selecting the correct TPI
all power screws are usually done for an particular aaplication. For static joints ie using screws to connect two flanges together or a similar application I use this rule of thumb to estimate the tensile load in the screw:-

F= T/(0.2*D)

where F= tensile force
T= torque on screw or bolt
D= nominal diameter of bolt or screw
and the 0.2 is the friction factor (with
no lubrication)

Bear in mind the friction factor can alter dramatically with
disimilar mating materials, lubricating oils or grease, surface finish of mating parts etc.


Hope this helps


regards desertfox