Guide Pin & Sleeve Material Questions

[Auspex]

Hello,
I have an old machine that employs a guide pin machined from 8620 steel that is case hardened to 50-60 Hrc that has been galling against a sleeve of 4140 hardened to 35-40 hrc. This is a recent occurance, and my research indicates that the carburizing process used to harden the shell may not be the best candidate here.

Should I consider using a different material altogether for the guide pin, or perhaps look at another heat treating process like nitriding or coating with something like titanium nitride?

I appreciate your advice!

 

[CoryPad]

You are asking the right questions and considering appropriate changes.

For us to help further, you would need to provide operating conditions like forces, temperatures, cycles, etc.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Auspex]

Cory,
Thank you for the response.

Here are some more details. Surface Ra on the pin is 8 uin (at least that's the spec anyway). If I had to guess, it probably looks more like 16. Operating temperature is probably 75-85 F.

Forces are a bit tougher to get a wrangle on. Under perfect alignment, the pins would only see axial force, and experience no galling. However, the level of misalignment has been tough to characterize. The pin has around 50000 lbf axial force acting on it during operation. Based on the bending stiffness of the pin and a guesstimate of misalignment, I calculate the pin to be exerting up to 800 pounds of force on the wall of the sleeve for pretty severe misalignment. Calculating resulting Hertzian pressure on the surface could be anywhere between 50 to 150 ksi depending upon the contact area....which I have little idea of the true area.

Again, these are back of the envelope type estimates. Some of these guide pins have seen hundreds of thousands of cycles with little pitting/galling. Some of the newer replacements are only making it < 10000.

I think the misalignments are exacerbated by a reduction in hardness that is inherent to the method of manufacture. Currently the pins were rough machined, case hardened, then finished. The heat treating, warps them and cleanup requires them to true up the pin and remove a significant portion of the case. This is what prompted me to look for other heat treating/coating methods. However, I am fairly inexperienced in this regard.

 

[TVP]

Does this pin/sleeve combination receive any lubrication? Galling due to high surface stresses sounds like a distinct possibility, especially if there is no lubrication. What is the length and diameter of the pin? Of the sleeve? Allowable runout of the two components? How much distortion is occurring after carburizing? Nitriding can reduce or eliminate distortion, so this may be a better option if the pin and sleeve are long. I would certainly investigate 1) lubrication, 2) nitriding, 3) increasing surface hardness of sleeve, and 4) low friction/high hardness coatings like TiN, DLC (diamond like coating), etc.

 

[Auspex]

Thanks TVP,

- It is a hydraulic application, so it is lubricated between the sleeve and pin.
- 3.996" OD, 22" length Pin
- 4.000" ID, 5.370" Length Sleeve
- Runout within 0.001"
- 0.005 - 0.015" distortion from carburizing. The case is spec'd at 0.06" (is it realistic to expect a reliable case this thick?). Either way, a fair amount of the case is removed during cleanup.


- What would you suggest for hardness of the sleeve?
- Which do see as more appropriate nitriding or TiN/DLC type coating? How much distortion can I expect from nitriding. From a galling perspective, which is more effective: thick case or hard/low friction coating...or both?
- Is there another solution I should consider like alternative materials, thru hardening, etc?

 

[unclesyd]

I would look at using D2 Tool Steel for both the pin and bushing. You can get the hardness you need, essentially above 58 Rc. Holds dimensions well during heat treatment. Outstanding wear resistance, especially if lubricated.

 

[NickE]

Unless this is a high production part (IE lots and lots of pins need to be made.) I like unclesyd's suggestion of D2.

Through hardening will get you away from troubles with grinding the case away, while The large carbide content of D2 will give you good wear resistance. Also there are lots of tool coatings (TiN, TiC/TiN, TiAlN, etc... --

http://www.ionbond.com)

that will give good lubricity and even more wear resistance.

 

[TVP]

Auspex,

I agree with the others-- if high volume production is not necessary, then tool steel may be a good choice. The parts are relatively big, so machining time and effort will be larger. 4140 should only be used if there is a requirement for toughness, and it would be at a maximum in the 35-40 HRC range. Increasing the hardness of 4140 to 58-62 HRC will result in very brittle parts, unless it is confined to the surface (induction hardening). Coating it with TiN, etc. would definitely increase wear resistance. Nitriding can essentially eliminate the distortion, because it does not involve the austenite to martensite volume transformation that occurs during quenching.

 

[Auspex]

Good information, thanks! The cost savings of reduced down time of the machine will definately outweigh the extra cost of the tool steel, so I think thi is a viable option.