TRW TAS85 Steering Gear Box Freeplay

[RichardEntrekin]

I am asking here, because this is a real gearhead question. I have some freeplay in my steering gear box, that translates into about one inch of freeplay at the steering wheel rim. It cannot be adjusted out with the sector adjustment screw. My question is "Is that normal?"

It is not in the steering column or u joints. The clamp is tight on the input shaft. I can lock down the input shaft and there is NO freeplay in the steering wheel.

For more info let me describe the lengths I have gone to in order to quantify this. First I mounted a dial indicator on the end of the pitman arm. I would have a helper move the steering wheel right then left until I detected 0.001 movement on the pitman. The helper would record the movement of the steering wheel. I loosened the sector adjustment screw until I knew the box was loose, I would tighten 1/16 of a turn and measure the movement in the steering wheel that produced detectable movement in the pitman arm. At a certain point additional tightening of the sector adjustment screw does not result in a further reduction of the steering play. I also know from on the road experiments that running the box too tight results in the steering sticking on small right and left movements.

I have also removed the drag link and adjusted the box according to the TRW manual. That is turning the sector adjustment screw in until no lash is detectable in the pitman.

You can laugh at my OCD recovering engineer behavior, but it turns out both methods of adjusting the sector screw have the sector screw at exactly the same place. So I am confident that is as good as it is going to get. Still, I have the inch of freeplay in the steering wheel that results in no movement at the pitman arm.

Is this normal?

[luvrbus]

You probably know the selector shaft lash is checked at the "center of travel" with the drag link removed checked by hand not using the steering wheel there should be no lash at the pitman arm if any it will translate to the free play on the steering wheel

[RichardEntrekin]

Cliff, you are absolutely correct. There is NO lash at the center of steering, with the pitman disconnected.

The problem is 1 inch of rotation in the steering wheel before any movement detected at the pitman. No adjustment in sector screw remedies this.

I can't believe that is normal.

[luvrbus]

Cliff, you are absolutely correct. There is NO lash at the center of steering, with the pitman disconnected.

The problem is 1 inch of rotation in the steering wheel before any movement detected at the pitman. No adjustment in sector screw remedies this.

I can't believe that is normal.  

No it's not normal you have something with play if you have no lash at the pitman arm the TAS 85 is a 0 lash sector,it could be the upper or lower mast bearings have a little play  if the u joints are tight

[Geoff]

It sounds to me like the worm and main gear are worn out and the freeplay is not adjustable.  You need a new steering box, or take a chance on a rebuilt box.  Some rebuilders don't put new gears in and you end up with a sloppy rebuilt box.

--Geoff

[RichardEntrekin]

Thank you both. I have looked at the exploded drawings and tried to figure out where slop could occur.

If I interpret the rebuild manual correctly, TRW has a procedure for setting the preload on one of the mast bearings.

I am worried about a reman box for the very reason you said.

If I could find a local place with a box, I could check it before buying. There is one in Tampa according to the interweb. I guess I am headed to Tampa.

Both of you were a great help in answering the question. I didn't think it was normal, but with a sample size of one, how do you know

[luvrbus]

Other than being annoying to you, it is well in the safety range for free play in the steering wheel according to the DOT rules

[Geoff]

A DOT inspection, when I was doing them, excludes power steering play.  The question I have is the 1" play with the engine running or with the engine stopped.  With the engine stopped is the true test with PS.  One inch play with the engine stopped with the lightest pressure on the steering wheel is under the 1.5" allowed.  When the engine is running you can't get an accurate measurement.

And if you go look at the rebuilt box take a pair of large vice grips to compare the play on both shafts on either box.  There should be no freeplay on either shaft.

--Geoff​

[luvrbus]

PS is not excluded now on DOT inspections and they give you a mile of leeway for free play more than I like 

[Geoff]

I quit doing DOT inspections​ because of the possible liability.  You never know who might have worked on the truck/bus after you inspect and pass it and if there is an accident they come after you as having passed it per DOT.  Not worth the risk for a couple of hours of paid work.

--Geoff

[RichardEntrekin]

Measured with engine off.

I talked with TRW tech. They say 1 inch or so on a 20 inch steering wheel is perfectly fine.

Annoying, yes that is accurate description.

I haven't given up. Finding a reman is challenging because the model number on the bottom is either missing or illegible. Newell can't tell me, and yeah I know they should be able to.

[Geoff]

For your benefit I just checked my freeplay and moving the steering wheel one way and slowly letting it settle with no hand on it in both directions I got 7/8" of an inch.

When I bought my bus it was at the time DOT inspections​ did not include the PS freeplay, and I had 3-4 inches of freeplay along with worn king pins (another item DOT inspections​ missed), and worn radius rod bushings (dog bones) (yup, another missed DOT item).  I was constantly correcting the steering when driving and it plain wore me out in 500 miles.  All that stuff is fixed now and the steering has been aligned now so it is a pleasure to drive.

--Geoff

[RichardEntrekin]

I am following up to report what I have learned, mostly the hard way, since I initially posted.

First, there has to be some inherent play in an integral steering gear box like the TAS series. To oversimplify, the input shaft is connected to a torsion shaft, the torsion shaft is connected on it's opposite end to the worm gear that moves the pitman arm when no hydraulics are present. When the torsion shaft twists, it starts to open ports which allow the hydraulics to step in. The key is that the shaft has to be able to twist a wee bit. Gearboxes come with different diameter torsion bars. Obviously the larger the diameter the less it will twist. The larger diameter will give a much better feel when dead on center, but you won't have very much power steering. It's a tradeoff.

TRW is not very forthcoming even when you have the serial numbers. They do not want individuals in that box.

So, after removing the box, and putting in new seals, and adjusting the sector gear to specifications. I had just over 1 degree of rotation of input shaft to produce detectable rotation the output shaft. Not too bad.

Two things of note. In the steering box, there are hard stops to prevent the torsion shaft from twisting too far. When we turn the wheel back and forth with engine off, we are feeling the steering hit those hard stops. Second, if you wish to quantify the amount of freeplay between the steering wheel and the pitman arm like I did, then the pitman must be disconnected or the front wheels off the ground. If not, you have the torsion shaft twisting. I didn't realize this until I took the box apart and noodled out how it works. So if the steering is loaded, the measurement is whacked.

But, I still had about 8 degrees (or 1 1/4 inches on a 20 inch wheel) of rotation at the steering wheel to produce detectable (0.002) motion at the pitman arm.

There is an intermediate shaft between the steering column and the box. I replaced the u joints which helped slightly but still had too much dead band in the steering. While playing with it I noticed the upper u joint "wobbled" when I moved the wheel back and forth. The lower bearing in the steering column was shot.

As you guys would know, both ends of the drive system have to firmly fixed for a double u joint system to be stable. That applies to driveshafts and also to steering intermediate shafts.

Newell used a tilt GM column, and it looks like the original bearing was crapola to begin with almost twenty years ago. So I am replacing with a higher quality bearing. If this one lasts another 20 years and 200k miles, I won't complain.

Thanks for all your help.

[TomC]

Even though my steering feels light and loose, on a straight road I can move the steering wheel 1/4" either way and the bus will move. I guess that means everything is tight. Good Luck, TomC

[Geoff]

I cannot help but to repeat that if you have worn radius rod bushings it doesn't matter how tight you steering is up front your bus is going to wander and you will be constantly correcting the steering.  The radius rod bushings keep the rear and tag axles from moving side to side.  Up and down on the bushings is okay, but side to side play will make your bus wander.  I just got a call from a friend of mine that is in the same boat I was in years ago-- everything was tight in the steering components and steering box but the bus still wandered.  He is now having his radius rod bushings replaced.

--Geoff