Posted by Jack on May 21, 1999 at 22:56:58:
In Reply to: Re: power steering/will orig. box take more torque? posted by Charlie on May 21, 1999 at 19:52:58:
Hi Charlie!
Can't help but think that we approached this subject a bit wrong-headed....think of your torque wrench....torque= applied force x length of the moment arm. The torque required to move the wheels is a quantified thing dictated by the truck, regardless of what method is used to make them move. The wheels don't know or care how you do this. The trick is to make the steering "effort" less for the driver, but since you can't get something from nothing, you lose motion to reduce effort at the steering wheel. In a recirculating ball power box, there is a sleight-of-hand trick being played. You turn the worm shaft, which would normally require enough effort to break loose the nut, thence turn the sector. The sleight of hand comes in when a valve senses your slightest movement of the worm, at which point a piston is forced against the nut, making it apparently easier to move. Still, the nut must only have sufficient force applied to it to move the sector, no matter where the "push" came from. The bottom line is that the "box" as an assembly will still be subjected to the same forces, but the worm shaft moves through a larger cycle to gain torque. When you add hydraulics, you are essentially "bringing in energy" from outside the "system" of the box, and you're substituting that "free energy" to replace what you would have needed to apply by hand. Since the hydraulics are outside the gear train, the box can now be designed to regain the increased path the wormshaft would have normally travelled without that "boost". In essence, this force is the same that you would have applied, but since the pump did it for you, you experience less effort. The cost for this is heat dissipation into the hydraulic fluid, and efficiency losses in the pump. Torque, as a component of the force expended in a simple machine, may be thought of similar to amperage in an electrical system. It's there on tap for your use, in the form of capacity, but the job dictates what amount is needed to overcome the "resistance", and that's precisely how much will be used, no more. if you use only enough to get the job done, that's all that flows through the circuit. IMHO, in the example cited here, no part of the steering box would be subjected to more "torque" than a manual steering box. That's because the load, the steering linkage and tires, doesn't require it. If you were to use a steering box for some other task, and the load at the sector shaft exceeded the norm for a steering system, then you'd have to apply more torque to make the sector shaft move. THAT would increase the force applied to the wormshaft. The answer is that any device can be used to apply force to the wormshaft, but no more will need to be applied than if you turned it by hand. The cited example should work, as long as the # of turns at the wheel doesn't make steering too cumbersome, and the pressure of the boost pump is regulated so that it can be controlled well. The floor is open.....