stainless steel or chromed steel

Friends,
I'm designing a gear drive for my robot. A center shaft for the gears was to be made from stainless steel. But out of curiosity, has
anybody used chromed steel for such application? Any drawback to such usage? Chromed should withstand wear better, correct? Any input will be much appreciated. Thanks.
Al
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snipped-for-privacy@yahoo.com (motorguy) wrote in

A chromed hardened steel shaft would last many, many times longer than SS. Stainless steel cannot be made very hard, therefore it's wear characteristics are poor in a shafting application. I would suggest only using SS for shafting in situations where it's chemical resistance properties are important. Note that the service life of a stainless steel shaft will be much shorter if the shaft is splined or keyed and has pulsing loads, such as a pump. I would suggest 8620, with a Case of 55-60 RC, then chrome.
If the shaft is splined, a preferable alternative would be H13 to 46-50 RC with laser hardening to 90-92 RC. Although the laser hardening is only 30-40 microns deep, the 90-92 RC surface provides a very long wearing, abrasive resistant surface. The 46-50 core hardness provides a good base of support for the laser hardened surface, but is still soft enough to be a little on the tough side instead of brittle.
(RC == Rockwell C scale)
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Anthony

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Hi Al:
Do you mean chrome plating or hard chroming? Chrome plating won't enhance wear resistance at all, though it can provide corrosion resistance (think paint). Hardface chrome is an undertaking and should only be necessary in high load, high abrasion applications. Unless you are running your robot in a wet or corrosive environment, I can't see any reason to use stainless or chrome either one.
Does this shaft ride in a bearing directly on the bearing rollers (like in a needle bearing), or is it just transmitting torque? If the gears are simply keyed to the shaft and the shaft rides in a mounted bearing with an inner race, wear should not be a concern. Ordinary carbon steel bar (like cold-drawn 1018) should suffice. If you are transmitting lots of torque (and you probably aren't) you might want to consider 1045 carbon steel or 4140 alloy steel. If you are indeed running the the shaft as a journal in a needle bearing, it should be surface hardened and ground. They usually recommend about 58 Rc. You can buy already-hardened-and-ground shafting from a number of sources, but forget about cutting keyseats in it, unless you have carbide tooling.
Actually the hardness and surface finishes are kind of anal for most applications. If you're not running the bearings at their rated load and you aren't running them 24 hours a day at rated speed, you can probably get by without hardening the journal. Better yet, just use an OI bronze bushing if you can.
Good luck!
Don

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In many robotic applications, power is of limited supply, thus there is a need to reduce frictional losses to a minimum. This could prevent the use of IO plain bearings, unless a pressure type lube system was employed. OI plain bearings would be cheaper though.
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Anthony

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Thanks guys, for the input. My design will be using the shaft as a bearing surface for the gear. The gear will be rotating wrt to the shaft. The rotation is slow, about 4 deg per sec. I'm not worry about wear too much. I believe the shaft will need to have a tight tolerance on the outer diameter. And I'm not experienced with chrome plating (the corrosion resistance kind) enough to know if it will can plated at a even and controllable thickness. Think .0005" thick. Any advice?
Al

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snipped-for-privacy@yahoo.com (motorguy) wrote in

Al, Flash chroming is very predictable, but nowhere near the tenths range. Whichever method you choose for chroming, and for this application I would suggest hard chroming, the shaft will have to be ground. For the application you state, I would definately add at least an OI plain bearing in the gear. To be effective, the chrome will need to be at least 0.1 mm thick, and even at this thickness, flaking is possible, more so with flash chroming than with hard chroming.
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Anthony

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from memory:
there are more kinds of stainless than there are satellite channels
start with 17-4 ph and check some of its charactersitics at various quench temperatures, and then check out the 400 series martensitics.
--Chromed steel is only as resistant as stainless if it the steel gets nickel plated before the chrome. --The coeff of friction of chrome is lower than steel, making it a preferred coating for hyd cylinders. Buit there is a little more to the efficiency gains than just swapping up -- wear depends more on pressure and upper limits of the materials (200psi al-stl, 3000 psi stl-bronze, etc) than the particular material --and use of chrome dip depends on your bearing - the tolerances from slapping a hot gob of chrome on a steel rod is not going to sit well with many bearings -
and then there's chrome on soft steel vs chrome on hardened steel.
best to find some nice ground and polished stainless shafting stock. all the hard work is done for you.
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Unless you're talking about hard chrome facing which is welded on and ground (and very expensive), the wear characteristics of ordinary electroplating are not so hot, especially if the shaft is soft steel. Imagine plating a loaf of bread with thin glass and then hitting it with a hammer.
What material you need all depends on your conditions. Consider these three main things:
1. Shaft subjected to high torque, bending moments, or heavy impact loads. 2. Shaft subjected to corrosive elements (water). 3. Shaft subjected to abrasive wear.
Unless your robot is seeing large loads like heavy machinery, then regular steel like 1018 cold-drawn rod is probably plenty strong. If you really are seeing high torques, then you might try 1045 carbon steel or 4140 or 4130 alloy steel, which can be ordered pre-hardened.
If you are running needle bearings directly on the shaft, you will need to have it case hardened. Generally, you want to start with a low-carbon steel if you're going to get it carburized. Alloy steel can be nitrided. Again, this is really only necessary if you're using needle bearings. Ordinary ball bearings or bronze bushings don't normally need a hardened shaft.
If you are using your robot outdoors and the shaft will actually get wet, especially salt water, you might want to use stainless steel. Stainless is getting expensive and scarce lately, so it might be a better idea to just find a way to protect the shaft from the elements. Stainless steel rod is most commonly available as #303 or #304. Those alloys should be plenty corrosion resistant unless you're going to subject you robot to acid! If you need stainless *and* you are going to use needle bearings, you will need to harden it. Unfortunately, most ordinary alloys of stainless won't harden well. There are a few hardenable grades though. #440 is the most common, but is relatively expensive.
I'd say avoid needle bearings and try to keep the water out of the mechanism and just use good old carbon steel bar if you can. You can have it black oxided or chromated if you're really worried, but in general, most machinery is simply bare steel and if you keep it lubricated it won't really rust that much.
Don Kansas City
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Stainless steel is steel containing chrome. The question does not really make sense. The question would be what stainless steel grade should you use (i.e. what % of chrome you want). You might mean steel with chrome plating instead of stainless steel? Plating and material selection is 2 things. You select the material for the structure requirement and corrosion resistance, than if you can't afford building a complete piece of stainless, you can do a plating over normal steel, or even painting. But plating could wear (as well as painting), and you have to be careful if the piece has to accept hits or would have to be machined after plating process, etc.. small holes might be completely filled by the plating process too, which would be undesirable.
stainless would be preferable in any case, but it might add lots on the cost... specially if that's for serial production.
I suggest you look into the mechanical handbook on stainless alloys properties.
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Mathieu Fregeau
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