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Corner break in Anodized spool valves in transmission controlHelpful Member! 

Mechie100 (Mechanical) (OP)
16 May 08 11:07
I have a automatic transmission control application with sliding aluminum spool valves running in a aluminum valve body.  I am having difficulty with controlling the corner break in the spool valve, after the Hard coat anodizing process.   The uneven corner profile creates a space for debris to collect and produces a sticky valve.   

Are there any solutions?

Is this the right forum for this discussion?
carnage1 (Electrical)
18 May 08 12:38
hard coat anodizing in my experience is about 2 thousandths thick, we used to re-ream critical holes after getting our parts back to true them and smooth them out. It wouldn't take off enough material to remove the anodizing, but it smoothed out the holes nicely.
Could you do an afterstep similarly to dress the outside of the valves and the inside of the valve body?
btw your reamers will become undersized quite quickly  from this so take that into consideration for whatever process you design.
patprimmer (Publican)
18 May 08 22:09
This area is as good a place as any I think.

The thickness of hard anodising can be controlled to some extent during processing. You can specify thickness up to a maximum. The maximum might be partly dependant on the alloy. I think the maximum I have seen is about 0.003" but that is from  30 year old memories so it may be inaccurate.

I am not sure it has anything to do with the anodising. It is hard to guess without seeing failed parts, but maybe the machining process is producing parts with:-

Bad fit.
A notch from machining tool tips in a critical area.
Poor original design without enough support for the load.



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Newmanite (Mining)
19 May 08 5:07
What temps are you running? Is the bore of the sliding spool also coated? Is it that the thermal expansion is creating the problem?How about the filtration?
Im only throwing out ideas maybe something you missed? Can you do trials where an alloy spool is used?Or a steel bore?An over length spool land?
Mechie100 (Mechanical) (OP)
19 May 08 11:48
The transmission is running at 100 deg C, with aluminum spool on aluminum valve body.   I do not expect relative thermal expansion to be a problem.    The valve bore is reamed.   

The filtration is set to 50 microns.   

I am pretty sure that the stickyness is from the corner radius (or the lack thereof).
Helpful Member!  btrueblood (Mechanical)
19 May 08 17:47
Mech, is what you are trying to say close to:  "after anodizing, the normally sharp machined corner becomes rounded"?  Sounds like a normal result of the anodizing process.  see

If the problem is that the dimension of the part varies at the sharp corner, could the corner be re-designed to have a controlled radius?

Would it be possible to re-machine or dress the corner in question to re-create a sharper corner after anodizing (i.e. rough machine prior to anodizing, leaving .015 inches or so extra "length", then anodize, then grind/mill the length to finish dimension).  Realizing this leaves the end of the spool un-anodized, but it doesn't seem to me that the anodizing is for corrosion resistance, as much as for a harder surface area along the wearing (cylinder) surface.  May be totally misreading the situation, in which case please ignore.
MikeHalloran (Mechanical)
19 May 08 19:30
Have you verified that the reported stickyness has anything to do with contamination?

I ask because in hydraulic valves, a lot of transient behaviors are critically dependent on the overlap/underlap between the spool and sleeve, and moving either edge by microns makes a difference.


Mike Halloran
Pembroke Pines, FL, USA

IceStationZebra (Mechanical)
19 May 08 21:26
50 micron filtration sounds a little "big". My experience is off-road hydraulic systems where 40 micron is quite common and my last project with electronically controlled valves was 10 micron. And the 40 micron machines are mostly open-center spool valves that are hand actuated.


Mechie100 (Mechanical) (OP)
21 May 08 13:41
I can see the uneven edge when I plot the profile.   
Btrueblood, is there any design solutions other than post machining.   

They tell me that the common filtration in automotive application is 50 microns.   Is this problem common in auto applications.

btrueblood (Mechanical)
22 May 08 12:39

"is there any design solutions other than post machining"

I don't know, but it used to be (I got out of Aerospace >10 years ago, and haven't really had to worry about aluminum since then), and so we just planned on post-machining for critical details.  But the web link I posted talks about the issue and claims that they can control the erosion/roughness of sharp corners by control of the electrical current (and presumably some of the chemistry details); I'd sure talk to them about it and see if they have any suggestions, maybe have them run a few parts and see if they can put out.   
swall (Materials)
22 May 08 15:57
I had some hardcoated 6061 master cylinder pistons polished/buffed in an attempt to obtain a smoother finish. Maybe that might get rid of your edge irregularities.
macquari (Automotive)
24 May 08 18:02
I agree with btrueblood on machining after anodizing due to uneven coating we anodize pins for mining applications and grind to bottom tolerance or lower to allow for specified thickness.
  I used to reclaim cranks using hardchrome and would build up to .010" o/size and grind back to finish size including radii.You may have a problem as others stated of the radii contacting before the faces,the spool body internal radii should be ultimately smaller than the spool radii.
  One other problem may be surface compatability Im not too sure about anodizing but with hardchrome you can't run chrome rings in chrome bores and some bearing materials were not suitable for chrome shafts.
Good luck I hope you solve the problem.  
carnage1 (Electrical)
24 May 08 20:08
is the valve anodising rubbing on anodising or anodising rubbing on aluminum?
Mechie100 (Mechanical) (OP)
30 May 08 16:18
The anodised spool is rubbing against the aluminum bore.   The valve sticks only about 5%  of the time.   

Mechie100 (Mechanical) (OP)
2 Jun 08 9:58
Thanks to all for the feedback and helpful hints.   
dm2000 (Automotive)
6 Jun 08 6:37
everywhere I've worked we've tried to keep the corner sharp in order to avoid sticky valve problems.  When we've allowed a radius, the valve became very sensitive to debris - think of a ramp that feeds little ball bearings up into the sliding interface... acts just like a one way clutch.
Tmoose (Mechanical)
6 Jun 08 8:14
Sometime in the 60s (?) Rolls Royce started using GM transmissions.  There was a story circulating that initially RR was disassembling them, finishing the innards properly, only to find in service the function suffered.  
I figured if the finishing included deburring the valve bits the story might just be true.  
pontiacjack (Electrical)
6 Jun 08 22:41
Tmoose- "Deburring" doesn't necessarily mean "breaking" (radiusing) of corner- important difference in terms.
I've worked on and used a number of those early Dual-Range GM Hydramatics that Rolls used into the sixties. My experience has been that the quality of the valve spools as they left GM was on a par with any more modern automatic I've had apart- very true and well-defined diameters, with neither corner breaks nor burrs. I do concede, however, that the precision of the valve bores in the valve body weren't held to as tight a tolerance as some later transmissions.
Tmoose (Mechanical)
7 Jun 08 20:05
>>>If<<< the story was true, and (even more doubtful) I remember the Car and Driver article correctly, RR did not do (whatever it was they did) for production cars because the transmissions did not work correctly.


Dan T

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