chrome plate thickness on hydraulic actuator piston

[brin]

Is there a typical thickness used for chrome plating on pistons for hydraulic actuators? I know QQ-C-320 specifies .002 min if not otherwise specified on the drawing, but I have another design reference that specifies .001 min. Which is more typical for an actuator subject to medium wear or exposure (eg: internal use)?

Thanks

 

[israelkk]

If you are referring to hard chrome plating then a lot more than 0.002" is recommended for wear applications. For worn crank shafts sometimes they use even up to 0.040" thick hard chrome for dimension buildup before regrinding the bearing seats to the original factory dimensions. Specifying 0.001" may be a wrong attempt to avoid large dimensional change due to the coating thickness and avoiding secondary grinding after coating. However, this thin coat is not enough for long time or heavy wear resistance.

 

[brin]

Thanks israelkk, is there any design guidance or reference material regarding how to specify the chrome plating thickness? The part in question is a piston rod (eg: seal surface) for a hydraulic actuator on an aircraft door.

 

[israelkk]

Hard chromium plating per QQ-C-320B class 2 type 1 or 2. On the shaft drawing place a dimension plus tolerance before plating and dinension and tolerance after plating and grinding. To my best judgement I would design the residual thickness to be not less than 0.004" after grinding. If the shaft is treated to strength equal or higher than 180000 psi then the grinding should be specified according to MIL-STD-866B titled "Grinding of chrome plated steel and steel parts heat treated to 180000 psi or over".

 

[TVP]

brin,

I do not work in the aerospace industry, so please take this as background info only: hard chromium plating is often specified at a thickness of 25 micrometers (0.001 inches) for general use hydraulic actuators/piston rods. By general use, I mean acutators/piston rods for automotive and industrial hydraulic dampers, gas springs, etc. The thickness requirements that israelkk indicates may be more prevalent in aerospace usage, although I thought these were only for really heavy applications like landing gear struts/actuators.

 

[israelkk]

TVP

This is interesting that for commerial use the typical thickness is 0.001 inch (0.025 mm). As brin (Aerospace) stated QQ-C-320 specifies 0.002 inch (0.05 mm) min if not otherwise specified on the drawing. Meaning that larger thicknesses are common or recommended in the military/aerospace applications.

 

[brin]

Thanks TVP and iraelkk- I went with .001 min for this part based on information in the OEM design manual for parts subject to medium wear or duty.

In general then, is the chrome plating thickness only a function of expected wear between overhauls? eg: the chrome plating surface is a sacrificial coating that can be renewed before any wear or scratch defects penetrate the base material? It is not a function of the load, speed, etc?

 

[Compositepro]

One consideration is cracking or flaking of the coating. Chome is very brittle so the thicker the coating the more likely it is to crack.

 

[brin]

Is there a guideline for max thickness?

 

[ThermalSprayer]

Hey Brin,

Have you given any thought to and HVOF coating instead of chrome? It has pretty much become the standard in the aircraft industry now, due to ecological reasons, cost, turn around time, and part life. An HVOF carbide coating is really tough and lasts much longer than chrome.

Regards,

Jim