Ratchet and Pawl Material Selections 1

[jeffc09]

I work for a company that has asked me to design a ratchet/pawl system to drive a chemical metering pump. For our first attempt, I found an OTS ratchet made from S45C Carbon steel (HRC 48-53), and to save money we planned on manufacturing our own pawl. For the first build we machined a piece of 303 SST (HRC 26.6 max) we had laying around to check the fit, etc. We expected it to fail, which it did in <5 hours of run time. The edge on the pawl was completely rounded out and would not grip the ratchet at all; the ratchet had no visible wear.

My plan is to switch our material to a metal with a similar hardness, and have found a local supplier for a heat treated 17-7PH which we can have made to our desired hardness. In our system, it would be ideal for the pawl to fail before the ratchet, as it can be rotated for use with the opposite side, so I was thinking an HRC of 46-48 would be ideal to preserve the ratchet.

Testing our competitors ratchet/pawl with nearly identical design showed that they are successfully using 12L14 material (extremely soft, HB 163) for both the ratchet and pawl (zinc plating added to the pawl). For our initial product it is out of our cost range to have a custom ratchet made with the added tooling costs, and was hoping to find a way to make our off the shelf ratchet work for us. The pawl can be machined in house and is easiest for us to adjust.

My main question is when selecting materials for a ratchet/pawl, what are the main design considerations to keep in mind? Tooth/pawl engagement angle, material selection, profile of the teeth on the ratchet (e.g. rounded over sharp), spring force? I have been trying to find material information online but haven't had much luck so far other than information pointing me to a machinist's handbook which I don't currently have. Any feedback would be appreciated, thanks.

 

[MikeHalloran]

You should have a personal copy of "Machinery's Handbook" if you are designing _any_ product.

... but I don't think it will help you much with ratchet/pawl geometry issues. For that, you may find help in a "machine design" textbook. ... but it will be limited, maybe a page and an illustration or two.

My experience, mostly limited to gritting my teeth while waiting for other people to design and redesign and redesign ratcheting mechanisms, says that it's easy. ... to sink a _lot_ of development time and a _lot_ of money in in homebrewed ratchet and pawl assemblies. They only _look_ simple.

I.e., I suggest that you buy both parts from someone who has already climbed the learning curve, and save money elsewhere.


Mike Halloran
Pembroke Pines, FL, USA

 

[TVP]

jeffc,

Material selection for a ratchet and pawl assembly depends, in part, on the following:

1. Forces on the mating pair- large static compression? High velocity as the pairs slip? Other?
2. Lubrication- Is grease allowed? If not, dry film lubricants/coatings?
3. Environment- Temperature, presence of moisture/water/chemicals, etc.
4. Cycling- How many, under what combination of forces, velocity, slip, etc.?

The ratchet and pawl systems with which I am most familiar require surface hardening in order to have sufficient wear resistance. S45C carbon steel (SAE 1045) at 48-53 HRC or sintered powder metals with similar effective hardness are two choices, with the latter requiring careful control of load paths (no tensile stresses allowed). Stainless steels, even high strength PH grades like 17-7 or 17-4, do not have good wear resistance under sliding contact conditions (gears, bearings, etc.), unless they are nitrided.

Was the competitor part (free machining steel, SAE 12L14) surface hardened at all? I am skeptical of such low hardness meeting any kind of cyclic requirement.