Power steering is a demand-based system which uses a servo valve to sense the application of torque to a steering shaft (from either end-by the driver or by the front wheels, which are identical in effect) and to proportionally direct hydraulic fluid so as to overcome or neutralize any forces opposing the driver.
The input and output shafts of the servo are connected internally by a small torsion bar. With the output end restrained, the input end can be twisted a few degrees to the right or left, and the torsion bar will cause it to spring back to a centered position when released (see figure 31). The output shaft is restrained by virtue of being connected (by the rack and pinion) to the steering resistance of the front wheels; The input shaft is "twisted" by the effort applied to the steering wheel. The greater the difference between the effort and the resistance, the greater the degree of "twist" momentarily existing between the input and output shafts as they rotate during steering.
The input and output shafts, in addition to transmitting steering torque, are also part of a concentric fluid metering valve, which opens during rotation in direct proportion to the degree of twist applied: if twist exists to the left, for example, fluid (provided by the pump) will flow so as to assist a left turn. The more twist, the greater the valve opening, and the higher the rate of fluid flow, until sufficient pressure builds up (in the cylinder) to overcome the resistance. If the twist is released (or if the resistance is overcome, which amounts to the same thing) the valve springs back to its centered position in which all fluid is permitted to exhaust:
The process is illustrated schematically in figure 31. In actual operation, pressure builds up and overcomes resistance within milliseconds. The twisting of the input shaft and the subsequent "catching up" of the output shaft and the release of tension occur, for all practical purposes, simultaneously and continuously.
The Woodward power steering system, as mentioned previously, uses interchangeable torsion bars. Changing to a stiffer torsion bar reduces the degree of twist resulting from a given torque at the steering wheel and, consequently, reduces the valve opening. This, in turn, reduces the rate of flow and, therefore, the rate at which pressure can build up to overcome resistance. The result is weaker response to the driver’s demand. Changing to a more limber bar allows a more rapid pressure rise, which results in a more powerful response to the driver’s demand. Remember that whether the demand originates with the driver (in the form of a hard pull on the wheel) or with the front end (in the form of contact with a rut), the effect on the power steering is the same—it will react in proportion to the demand, regardless of the source.
Much of the demand placed on the system in a race car takes the form of impulsive loads, as opposed to the relatively steady loads of street driving. These loads are due partly to the broader range of rapid motion employed by the race driver, which can twist a stock servo valve open to its limit stops in the course of overcoming the weight-jacking burden of positive caster.
Positive caster increases the turning resistance of the front wheels, and does so progressively. With zero caster there is practically no difference in turning effort between a straight-ahead position and full lock, whereas with, for example, six degrees positive caster there is a very noticeable increase, or surge, in muscular effort as the wheel is turned. This resistance is multiplied by quicker steering gear ratios. The stiffer-than-stock bars commonly installed in GM-based servos do not attenuate this rapid rise in steering effort very far beyond an initial movement of the steering wheel. It is the ability to reduce this muscular surge to a comfortable level over the entire range of movement of the driver’s arms that distinguishes power steering designed for race cars from that designed for street automobiles.
The choice of torsion bars available to accomplish this is extensive, as can be seen in figure 15, which shows the resistance curves of the various bars.
The limiting factors for bar choice are:
(1) If the bar is too stiff for the amount of caster in the front end, it will become hard to turn the wheel near the end of the steering stroke. This will make it difficult to keep from spinning out under loose conditions, because precise control will not be available near full lock. Controlling the car’s normal rotation into a slide under tight conditions may be possible, but tiring.
(2) If the bar is too soft for the amount of caster in the front end, it will overcome too much of the steering resistance and the driver will not have enough feedback to gauge how far the wheels are being turned (in this case, the driver gets nervous rather than tired).
Quick rack and pinion ratios have less mechanical leverage than slow ones, and this has to be factored into the steering resistance. However, the effect of the ratio is fairly constant (rather than progressive, as with positive caster) and can be dealt with by tailoring the baseline system pressure until a comfortable starting point is reached (see "System pressure").
For more information:
| USA:|| Woodward Machine Corporation|| |
| || Contact:Tony Woodward|| |
| || 3592 Burd Road || |
| ||Casper, WY 82604 || |
| || USA|| |
| || Tel. +1-307-472-0550 || |
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| Europe:|| Hydroline Powersteering BV|| |
| || Contact: Rini Ruitenschild|| |
| || Gaffel 11|| |
| || 3891 KC Zeewolde|| |
| || The Netherlands|| |
| || Tel. +31 - 36 538 72 80|| |
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