Tom Wilson
June 4, 2012

In a simple overview, imagine a thick-walled steel tube, about ¾ of an inch in diameter, in your left hand. One end of the tube has two flats machined into it. Now imagine a second shaft in your right hand, this one with a female socket end machined to accept the flattened end of the first shaft. The two shafts mate, but the clearances between the flats on the first shaft and the matching female socket's flats are loose. Put together, you can easily rotate the two shafts independently of each other a little bit because of the sloppy fit. But after about 15 degrees of free rotation the flats come together and the two shafts go solid.

OK, now we add a torsion bar to our mental assembly. The "T-bar," as the chassis engineers always call it, is a solid metal rod about the size of a pencil. It's firmly fixed to the shaft in your right hand, the one with the female socket. When the two shafts are slipped together, the T-bar slides through the hollow center of the other shaft, the one in your left hand with flats on the end. Once the two shafts are mated, a hefty dowel pin is driven through the shaft in your left hand and the end of the T-bar. This joins the T-bar to both shafts, and you can no longer easily rotate the two shafts independently of each other; the slop is gone.

But, as you've guessed, the T-bar is really a spring. Put enough leverage on the shaft in your left hand and it will still move relative to the second shaft, at least until the flats come in contact and the assembly goes solid.

If you've ever tried to coast your Mustang down a twisty road with the engine shut off, you've noticed how the steering feels--besides needing high effort, strangely gummy. You grunt the steering wheel a few degrees and nothing happens. Then all of a sudden the car turns and the effort drops sharply. Try to steer in the opposite direction and you go through the same sensation of winding a gigantic rubber band--that's the T-bar twisting followed by the flats finally going solid.

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So, we've got our two shafts mated and pinned together via the T-bar. Now we'll add the final piece, a collar that slips over our shaft assembly. The collar is intricately machined and, among many features, has a shoulder in it that butts against the female socket on the shaft in your right hand. A dowel pin fixes the collar to that same shaft in your right hand so the collar is really part of that shaft.

Now, the collar has no locating function: It doesn't lock the two shafts together or anything like that. Its job is to form a valve in conjunction with the shaft in your left hand. Remember, the collar is pinned to the right shaft, but forms a valve with the left shaft.

The valve is formed by machining troughs into the ID of the collar and the OD of the shaft. Power steering fluid is pumped to this area and flows between the shaft and collar. As the two shafts rotate independently of each other--in the "slop" that's dampened by the T-bar--the area open to the power steering fluid changes as the shaft and collar rotate independently of each other. This ports, or controls, the amount of power steering fluid, and thus, the amount of steering assist.

And for the record, the shaft in your left hand is the steering rack assembly's input shaft; the shaft in your right hand is actually the pinion. At its other end, the right shaft is milled with gears that mate with the steering rack.

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