Type of steel for a spool valve

Jimmy Wilson · Aug 31, 2019

I need to make some new "spools" for a cessna spool valve. I was thinking about using stainless so that I wouldn't have to worry about rust/corrosion and the pitting that lets oil pass by. Not looking for a scolding, but I used some #7 rebar 4 years ago for 2 spools that still work great. The rebar was probably a poor choice for a few reasons, but the problem I had was different levels of hardness most likely from the manufacturing process. It took quite a while to get the wavy bar down to a .6247-.6249 blank. I was thinking 1045 might be a good choice, but if I knew for sure, I wouldn't be here asking. If stainless is a bad idea for some reason, please let me know.

gbent · Aug 31, 2019

If you have enough water in your hydraulic oil to need a stainless spool you have bigger problems than you have mentioned.

Red James · Aug 31, 2019

You just need to use a wear resistant steel, like 4140 PH. Corrosion is not gonna happen.

DanielG · Aug 31, 2019

The guy I knew who made spool valves used 440C. I'm not sure if it's the same style of spool valve; his were ridiculously tight clearance, lapped, air gaged, etc.

From an ease of machining and surface finish standpoint, the 4140 PH sounds like a good idea. 17-4 might also be a good option, who knows.

Illinoyance · Aug 31, 2019

My understanding is the spools are lapped to precise size. They might even us selective assembly to match spool diameter to bore diameter.

Milland · Aug 31, 2019

If it was me, I'd want to use a steel with anti-galling properties as well as hardness. But especially if the manifold or valve body is an aluminum, I'd want a high-polish on any contact surface to try to minimize bore wear. I think an appropriate alloy steel with a gas-nitriding treatment after hardening would be great.

Gas nitriding - Case hardening without subsequent hardening operation

On the rebar? If it's good enough for bridges, it's got to be fine for flight purposes, right?

Oh, wait...

Bridge Rebar Corrosion

Jimmy Wilson · Aug 31, 2019

Thanks to all. 4140 sounds good to me. The body is cast iron and didn't seem to have any pitting from what I could see with a loop.

Lapping wit matched fir would be nice, but that kind of precision is less than once a year for me. I got it to within .0005in and got the rest with 220 400 600 1000. Took me about an hour for each of them, but that's all I had to work with. I don't know if I got lucky, but whatever the clearance was, it takes the bucket a week to come down to the ground where it took about an hour when I got it. I know it shouldn't allow any movement/flow at all, but it was more than adequate for my needs.

I don't know where the corosion came from, but it was an old D100 backhoe that had been sitting for years. It was still operational, but when using it to load something heavy, by the time I'd get over to remove the chain, the bucket would be resting on whatever I had just set down.

Bill Simmons · Aug 31, 2019

I assume that one end of the spool exits the spool valve assembly to connect to some sort of lever to push and pull the spool. And this is where rust and or corrosion is most likely to occur. If there is some sort of boot that will keep water out then stainless not needed. However, if the rod is exposed to the elements then it will rust and corrode and eventually begin leaking at the seal. Most the spools I have seen stay pretty shiny suggesting they are stainless.

I just looked at 7 spools on my backhoe (30 years old) and 2 on my log splitter and there is no rust on any and they are exposed. They are magnetic ruling out some stainless. So I say they are either stainless or plated.

john.k · Aug 31, 2019

IMHO,spools are often quite soft steel ,hardchrome plated.......backhoe spools often have springs ,plungers and reliefs inside the spool........very few are a simple solid spool.........I have had spools hardchromed oversize ,and then honed out the valve body to fit.....on many excavators ,spools are fitted "tight",which prevents dirt from entering the clearance ,and are hydraulic pilot operated.

DDoug · Aug 31, 2019

I'd stick with the Re-bar...seemed to work good for this long.

DDoug · Aug 31, 2019

john.k said:
IMHO,spools are often quite soft steel ,hardchrome plated.......backhoe spools often have springs ,plungers and reliefs inside the spool........very few are a simple solid spool.........I have had spools hardchromed oversize ,and then honed out the valve body to fit.....on many excavators ,spools are fitted "tight",which prevents dirt from entering the clearance ,and are hydraulic pilot operated.

I suspect this one (being very old) might have "O-rings".

Jimmy Wilson · Aug 31, 2019

digger doug said:
I suspect this one (being very old) might have "O-rings".

Mine did have O-rings and whatever metal was used, it was somewhat resistant to rust and was not plated. On the bottom was a spring with a washer and c clip.

hanermo · Aug 31, 2019

#7 rebar comes out in google at 22 mm +-.
If rebar lasted 7 years, drill rod or similar will last longer and be better to start with.

The 400 series stainless steels should last longer than rebar, and for these sizes might cost 50$ +-.
Stainless is generally less strong, less resistant, and much more prone to galling.

The posts re hardening, carbides, nitriding, etc. are all correct, ime, but if rebar lasts 7 years then *anything* quality should last longer.
Per pics, I would make the part from
1. linear guide rod aka thomson rod if near-size is available
2. prehard anything steel == drill rod == silver steel
3. 4xx series or 316l series stainless

No idea what a cessna spool valve is or how it wears.
But generally items that wear should not be stainless.
Or if stainless, use complex and costly sleeves, pads, ht, etc, or very very expensive and exotic steels like inconel.

I imagine the stainless part lasting about 3-7 years vs rebar, as-is, from common stainless stock.
303, 304, 316, depending.

john.k · Sep 1, 2019

I would check the cylinder packings,and the cylinders too,for wear spots and corrosion.....you always get wear areas where the bucket skims the ground,and often at half lift too,if its driven long distances,as in ,on the road.Replace the packings on the piston,might solve the leakdown.New V section ring filled packings take up large amounts of cylinder wear.Im looking in the F550/750 ford manual,and the packings are the old black hard chevron type,dont take up much wear.Thats an English Ford ,Tho.

Jimmy Wilson · Sep 1, 2019

john.k said:
I would check the cylinder packings,and the cylinders too,for wear spots and corrosion.....you always get wear areas where the bucket skims the ground,and often at half lift too,if its driven long distances,as in ,on the road.Replace the packings on the piston,might solve the leakdown.New V section ring filled packings take up large amounts of cylinder wear.Im looking in the F550/750 ford manual,and the packings are the old black hard chevron type,dont take up much wear.Thats an English Ford ,Tho.

Thanks, that makes sense. With three identical cylinders on the Case D100, swapping with 1 that doesn't move would tell me if it is valve or cylinder related. I keep it pinned up when not in use to take the pressure off the hydraulic system, so for now, the minor flow isn't a problem.

ratbldr427 · Sep 2, 2019

You may know this ,but maintain the sharp edges on the spool segments to help stop trash ingress which causes sticking;don't radius the edges.

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Type of steel for a spool valve

Jimmy Wilson

Plastic

gbent

Diamond

Red James

Stainless

DanielG

Stainless

Illinoyance

Stainless

Milland

Diamond

Jimmy Wilson

Plastic

Bill Simmons

Cast Iron

john.k

Diamond

DDoug

Diamond

DDoug

Diamond

Jimmy Wilson

Plastic

hanermo

Titanium

john.k

Diamond

Jimmy Wilson

Plastic

ratbldr427

Titanium

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