I have an old Pettibone Extendo 88 forklift that I am working on some hydraulic issues this winter. I have rebuilt the Char-Lynn SCU unit, but have yet to be able to test it out. When I got the machine the steering worked, but the SCU whined & growled like crazy, even if I wasn't touching the steering wheel. When I took it apart, I didn't see any internal damage, here are some pics: http://imgur.com/ZyvyCo3 http://imgur.com/MTYTbTH http://imgur.com/B7yaN2Q http://imgur.com/FFNKLUv http://imgur.com/QH6S5UQ http://imgur.com/Yd2ySx7 One thing I noticed when I was disassembling is that there was some wear on the endplate where the rotary valve makes contact. The endplay of the rotary vale was almost 0.012". That seemed to me like oil under pressure could be bypassing that clearance and causing the noise? I ground and lapped various parts to get them square and free of any wear marks, when I was done the endplay was about 0.003". I'm hoping that is enough clearance so when the unit heats up and thermal growth sets in, the unit doesn't lock up. If it turns out this unit still doesn't work properly and still whines etc, I'd like to replace it with a new Eaton unit. The problem is, I don't know what model/size Eaton to replace it with. I don't know the GPM of this unit. I'm wondering if anyone has any experience with hydraulic SCU's and could tell me what the replacement is for my old Char-Lynn unit? The Model number is UM3-01, the S/N is 1285-
Make sure when you put the valve back in that you are clear of the steering wheel when you start the engine. There is a procedure for timing the spool to the barrel and if wrong instead of the component being a valve it becomes a motor. The wheel will turn to the lock and if you hand is in the way it will turn you arm into a piece of twisted licorice. You can check with most any hydraulic suppler to get the specs you need. Out here I deal with Spencer hydraulics or Hydraulic Engineering. I'm sure there will be someone around you.
Thank you for the safety advice, I will mount it up without the steering wheel initially. When I was rebuilding the unit, I used an Eaton series 6 manual as a reference. It did call out for spool timing marks, but I didn't see any marks on the spools I was dealing with. Also, some of the parts they were calling out in that service manual looked different than what I had I'm my Char-Lynn unit. When I was using the pin to align the spools, I didn't see any differences in the ports if I had the spools at 0 degrees or 180 degrees. I will find out if I guessed right or wrong when I put the hyd pressure to the unit. I did locate the nearest Eaton/Char-Lynn distributor, they are looking into the model and serial number I gave them, hope then find some specs.
The ones I've done in the past would be still until you started to move the steering wheel. Once it moved it was wind up time. There was a procedure for lining up the cross springs in the bottom of the valve. I remember something about lining up a set of holes in line with the springs. It took a few tries that first time to figure it out. I wasn't too smart back then so didn't think to mark the housing and spool position when I took it apart the first time. I get a little smarter with each mistake. Now days a little dumber all the time as I forget what I used to remember.
This usually points to internal leakage across the rotary valve/end plate or incorrect centering/timing, so the unit is never truly in neutral and is always trying to meter oil. The wear you found at the end plate and the large endplay you measured (around 0.012") are exactly the kind of conditions that let high‑pressure oil leak where it should not, which can both create noise and reduce steering stiffness/feel; reducing that to about 0.003" while keeping surfaces flat and smooth is in the right direction, as long as there is still enough clearance for thermal expansion so it does not bind when hot. Verify timing/centering of the spool to the commutator (as @John C. warned, a mis‑timed unit behaves more like a motor than a valve and can self‑steer hard to one lock on start‑up), then pressure‑test the SCU with a small test pump and gauge to see whether it achieves rated pressure without excessive leakage or noise. If it fails that test or still makes excessive noise, use the UM3‑01 data (displacement in cc/rev or in³/rev, maximum pressure, porting style) to select an Eaton replacement with the same or very close displacement so you keep about the same turns lock‑to‑lock with your existing cylinders and pump. A good hydraulic shop or Eaton/Char‑Lynn distributor can usually cross‑reference an older UM‑series number to a current series 10/20/40 unit. If you want a more structured way to pick a replacement size, there is a 7‑part hydraulic steering series that walks through exactly this process (SCU basics, displacement, cylinder sizing, turns lock‑to‑lock, etc.) using off‑highway/mobile equipment examples very similar to your Pettibone; it is written for off‑road gear but the same logic applies directly here: https://epichydraulic.com/steering/ You can also plug your pump flow, SCU displacement and cylinder dimensions into this free steering calculator to check how many wheel turns you should get and how a different Eaton size would change steering feel: https://calculator.epichydraulic.com/calculators/steering/hydraulic-steering-calculator