TRW TA S40 Series, Service Manual

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GROUP 0603-V-003-0207 STEERING
7
DocID TRW TAS STEERING GEAR Page 1-9
Review Draft
8
General Design
Integral Power Steering
TAS power steering gears are another part of our fully integral
hydraulic power steering gear family. Fully integral hydraulic power
steering means that the gear contains a manual steering mechanism, a
hydraulic control valve, and a hydraulic power cylinder, all in a single,
compact package.
Rotary Control Valve
The rotary control valve combines simplicity of construction with desir-
able performance characteristics. The speed at which the driver can
turn the steering wheel with power assist is dependent upon the pump
flow (measured in gallons per minute or liters per minute) directed to
a cylinder cavity. The control valve controls flow through the steering
gear.
The pressure (measured in pounds per square inch, or bar) required for
the gear to steer the vehicle is created by the power steering pump to
overcome resistance at the steered wheels. The control valve senses
these requirements and directs fluid to the appropriate cylinder cavity
in the steering gear (and in the auxiliary cylinder if it is a dual steering
system) at the proper flow rate and pressure.
Pressure Means Work, Flow Means Speed
The higher pressure a steering gear can withstand, the more work it
can perform. The maximum operating pressure for TAS40, 55, 65 and
85 gears is 2,175 psi (150 bar). The maximum operating pressure for
a TAS66 gear is 2,320 psi (160 bar). Maximum flow rate for all TAS
gears is 8 gal/min (30.3 L/min).
The TAS series gears can steer a vehicle within its front-end weight
rating through a turn at low speed and engine idle. As the driver turns
the steering wheel faster or slower, more or less fluid will be required
by the gear. TAS series vehicle front-end weight ratings are as follows:
Gear Front Axle Rating
TAS40 9,000 lb (4,100 kg)
TAS55 12,000 lb (5,400 kg)
TAS65 14,300 lb (6,500 kg)
TAS66 14,300 lb (6,500 kg)
TAS85 18,000 lb (8,200 kg)
The recommended minimum flow at 1
1
⁄
2
steering wheel turns per
second is as follows:
Gear Minimum Flow Rate
TAS40 2.2 gal/min (8.3 L/min)
TAS55 2.6 gal/min (9.8 L/min)
TAS65 3.0 gal/min (11.4 L/min)
TAS66 3.0 gal/min (11.4 L/min)
TAS85 3.6 gal/min (13.6 L/min)
If the steering gear valve is controlling an auxiliary cylinder, increased
minimum flow is required (generally at least 75%) based on the size of
the auxiliary cylinder and the vehicle's steering geometry.
Maximum internal leakage for all TAS gears is 1 gal/min.