Fixed Power Nut Material - Using rec. to not mate like components threads.

[johnchrc]

Fellow Engineers,

I have a metal selection question based on my companies proclivity to use dissimilar metals wherever a thread connection exists except perhaps for low alloy steel. But CRA and SS material it is a rule of thumb we live by. I have a 304 SS Reservoir with low hardness and fine SA box threads at each end. I'm installing a Fixed Power Screw Nut of unknown material. It will be Box x Pin with a 1-1/4 UNC 7 TPI inner Power Screw Box thread. The Screw will be steel 38-45- HRC.

The Head Actuator material that makes up to the Fixed Power Screw Nut will be Tough met and have the 1-1/2-16 SA Pin so the Fixed Power Screw Nut will have a 1-1/2 -16 SA Box up.

Question 1) Can you make-up tough met materials together without galling problems? Then there would be no need for a third material. Could I use tough met for the Power Screw Nut?

Question 2) Assuming tough met x tough met isn't the best choice; I need a lubricating material for the Fixed Power Screw Nut. Would wrought phosphorus-bearing bronze PB-102 Grade UNS C51000 to ASTM B-139 46 ksi MYS and 55 UTS be a good choice? ------ Or ---- Ampco 45 Nickel Aluminum Bronze – UNS C63000 – 65 ksi Yield Strength, 105 ksi UTS 10% elongation

Question 3. Would another material choice by preferred over the two I have selected? I have listed some materials I like that have the strength and lubricity to keep my screw-nut friction down below .15. I feel the UNSC5100 phosphur bronze would be better for low friction and thus torque but the UNS C63000 Ampco 45 would be stronger and sufficiently slick for the Screw.

I'm looking for direction on the Middle Fixed Power Screw Nut. One side makes up to the Head Actuator made of tough met and the other to a 304 SS Reservoir, and on the ID a Power Screw made from steel with std black finish oil lubricated specially made 1-1/4 SHCS


Material Properties
Spinodally-strengthened copper alloy (UNS C72900) bar (Tough met)
Minimum Yield Strength 110 ksi; Minimum Tensile Strength: 125 ksi Minimum Elongation: 10% Hardness: 27-36 HRC

304 SS
Reservoir with 1.5000 SA 16 TPI SA b0x threads and o-ring seal at 1.564 with Major Part OD 1.69 OD.
----------Yield Strength-----------------Tensile Strength-----------------Elongation -----------Reduction of Area2-------Hardness Rockwell C
Min.-------30.0 ksi (205MPa)-----------------75.0 ksi (515Mpa)----------------30% -------------------50.0% Max. ------- --------------
Max.--------Note 3---------------------------110.0 ksi (760)--------------------------------------------------------------------22.0

Note 1: Yield strength to be 0.2% offset for bars and 0.5% by extension for tubulars.
Note 2: For tubular products, there is no requirement for reduction of area
Note 3: The maximum yield/tensile strength ratio shall be 0.85.

Phosphorus-bearing bronze PB-102 Grade UNS C51000 to ASTM B-139 46 ksi MYS and 55 UTS

Ampco 45 Nickel Aluminum Bronze – UNS C63000 – 65 ksi Yield Strength, 105 ksi UTS 10% elongation

I would appreciate any suggestions for materials based on galling prevention and minimizing Power Screw Thread friction. My hard drive crashed so I'm on an old laptop or I would send picture. It really is just two connections and three parts when the screw insn't considered. However, it should be just in case friction becomes very high.

Thanks You,

CJ

- CJ

 

[desertfox]

Hi
This is not an easy set of questions, material selection for power screws and nut combinations depends also on the loads, screw speed, enviroment and forced lubrication or just manually greased at a frequent periods. We typical run power screws of low alloy tough steel into various brass, phosphor bronze nuts, the loads are very high as are the speeds and so we have to pump oil into the joints to lubricate it and keep it cool, my advice would be to talk to a tribology engineer or a local engineering university.

 

[TVP]

I think you should contact Materion directly to discuss this issue. In general, steel external threads mate well with copper alloy internal threads. I am not familiar with Toughmet being used in externally threaded applications, and have some reservations about doing so. I think any of the copper alloys that you mentioned, Toughmet, phosphor bronze, or Ampco 45 would be acceptable as the nut member against a steel screw.

 

[johnchrc]

Thanks all for your responses.

Note: the Power Screw will use a 1/2" ratchet drive x 1 ft long with allen head
the threads will be lubricated but will see use every day x 3

I attached a pic of the lower cutter head half which worked on both motor torque and pressure.

Regards,

CJ

- CJ