Marles Varamatic
Encyclopedia
Marles Varamatic is the trademark name of an automotive power steering
system of the 1960s and 70s, which was characterised by its variable steering ratio
. It was developed in 1961 by the Adwest Engineering Co Ltd of Reading, England, in conjunction with the Bendix Corporation
. The Adwest product was fitted to several British luxury automobiles of the era, including; the Aston Martin V8
, Daimler Sovereign
, Jaguar 420 and Rover P6B 3500S
.
.
piston operating in a cylinder integral with the steering box casting. The teeth of a rack projecting from the piston mesh with a sector of a spur gear machined into the sector shaft. A roller in the sector shaft meshes with the helical track on the cam, and the two together are responsible for providing the variable ratio. No external lockstops are provided as they are incorporated into the steering box.
When steering effort is applied at the wheel it is transmitted to the rotor, which transmits the effort to the hour glass cam by means of the torsion bar. The rotor is normally centred in its sleeve by the torsion bar but manual steering effort twists the torsion bar, permitting the rotor to turn within its sleeve, opening the way for hydraulic fluid pressure to be applied to one or other side of the servo piston, thus assisting in turning the front wheels.
recirculating ball system that was standard fitment on the 420, and 3-1/4 turns for the constant ratio Burman power steering system optional on the 420's predecessor, the Jaguar S-Type.
Whilst advanced for its time, the Varamatic system did have certain drawbacks. It was more complicated and expensive than a constant ratio power assisted system. It could also catch drivers out when they attempted to "dial in" just the right amount of steering correction in reaction to oversteer, the non-linear response of the steering working against the driver in such circumstances.
The Varamatic system was also criticised for feeling too light at high speed, when greater weighting tends to give the driver more confidence in making fine steering adjustments. Later systems by other manufacturers would adjust the behaviour of the steering in reaction to road speed.
Power steering
Power steering helps drivers steer vehicles by augmenting steering effort of the steering wheel.Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver needs to provide only modest effort regardless of conditions. Power steering helps considerably when a...
system of the 1960s and 70s, which was characterised by its variable steering ratio
Steering ratio
Steering ratio refers to the ratio between the turn of the steering wheel or handlebars and the turn of the wheels .The steering ratio, is the amount of degrees you have to turn the steering wheel, for the wheels to turn an amount of degrees.In motorcycles and bicycles, the steering ratio is...
. It was developed in 1961 by the Adwest Engineering Co Ltd of Reading, England, in conjunction with the Bendix Corporation
Bendix Corporation
The Bendix Corporation was an American manufacturing and engineering company which during various times in its 60 year existence made brake systems, aeronautical hydraulics, avionics, aircraft and automobile fuel control systems, radios, televisions and computers, and which licensed its name for...
. The Adwest product was fitted to several British luxury automobiles of the era, including; the Aston Martin V8
Aston Martin V8
The Aston Martin V8 is a two-door coupé automobile manufactured in the United Kingdom from 1969 to 1989.Aston Martin's customers had been clamouring for an eight-cylinder car for years, so Aston Martin designed a larger 2-door saloon for V8 applications. The engine was not ready, however, so in...
, Daimler Sovereign
Daimler Sovereign
Daimler Sovereign was a name applied by British manufacturer Jaguar Cars to a sequence of luxury automobiles built by it but carrying the Daimler badge between 1966 and 1983....
, Jaguar 420 and Rover P6B 3500S
Rover P6
The first P6 used a 2.0 L engine designed specifically for the P6. Although it was announced towards the end of 1963, the car had been in "pilot production" since the beginning of the year, therefore deliveries were able to begin immediately. Original output was in the order of . At the...
.
Overview
The Varamatic is essentially a refinement of the power assisted "cam and roller" type of steering, being referred to instead as "hour glass and roller". The steering box contains an "hour glass" cam machined with a varying helix angle to provide the variable steering ratio. In common with other power steering systems, control of power assistance in response to steering inputs is via a hydraulic control valve in the input shaft of the cam. Pressurised hydraulic fluid is supplied to the system by a separate hydraulic pumpHydraulic pump
Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic.Hydrostatic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement cannot be adjusted, or variable displacement pumps, which have a more...
.
Steering box
The hydraulic assistance is supplied by a servoServomechanism
thumb|right|200px|Industrial servomotorThe grey/green cylinder is the [[Brush |brush-type]] [[DC motor]]. The black section at the bottom contains the [[Epicyclic gearing|planetary]] [[Reduction drive|reduction gear]], and the black object on top of the motor is the optical [[rotary encoder]] for...
piston operating in a cylinder integral with the steering box casting. The teeth of a rack projecting from the piston mesh with a sector of a spur gear machined into the sector shaft. A roller in the sector shaft meshes with the helical track on the cam, and the two together are responsible for providing the variable ratio. No external lockstops are provided as they are incorporated into the steering box.
Control valve
In common with other power steering systems, the control valve is of the rotary type and consists of a valve rotor and torsion bar. The valve rotor is the input shaft to the steering gear and has six longitudinal grooves machined into its outer surface. When no load is applied to the steering wheel, these grooves lie between six grooves in the valve sleeve and no hydraulic assistance is applied.When steering effort is applied at the wheel it is transmitted to the rotor, which transmits the effort to the hour glass cam by means of the torsion bar. The rotor is normally centred in its sleeve by the torsion bar but manual steering effort twists the torsion bar, permitting the rotor to turn within its sleeve, opening the way for hydraulic fluid pressure to be applied to one or other side of the servo piston, thus assisting in turning the front wheels.
Pressure pump
The hydraulic pump is of the vaned rotor type, typically of Saginaw manufacture, and is connected to the steering box via flexible feed and return hoses. The fluid reservoir is incorporated in the pump's end cover. Typical system operating pressures are in the range 7.8 - 8.8 MPa, the operating medium being automatic transmission fluid.Operation
The varying pitch of the cam results in the variable steering ratio, with the lowest gearing being at the straight ahead, rising rapidly to either lock. The rise in gearing (which in the Jaguar 420 was equivalent to a drop in ratio from 21.6:1 to 13:1) occurs almost entirely within the first half turn of the steering wheel from the straight ahead position. The effect is to give very light and relaxed steering at the straight ahead, with increased reaction when cornering.Benefits and drawbacks
The major benefit of the Varamatic system is the reduction in the amount of "arm twirling" and effort required when steering, particularly when a lot of cornering is involved. For example, the Varamatic system fitted as an option to the Jaguar 420 required only 2-7/8 turns of the steering wheel from lock to lock, which compares favourably with 4-1/4 turns between locks for the manual Burman F.3Burman and Sons Ltd
Burman and Sons Ltd, of Ryland Road, Birmingham, England, manufactured Burman-Douglas steering gear.Their recirculating worm and ball design of steering gear was fitted to pre-war vehicles such as the Ford Eight and the Ford Prefect, the Bedford CA, plus many other vehicles - both pre and post-war....
recirculating ball system that was standard fitment on the 420, and 3-1/4 turns for the constant ratio Burman power steering system optional on the 420's predecessor, the Jaguar S-Type.
Whilst advanced for its time, the Varamatic system did have certain drawbacks. It was more complicated and expensive than a constant ratio power assisted system. It could also catch drivers out when they attempted to "dial in" just the right amount of steering correction in reaction to oversteer, the non-linear response of the steering working against the driver in such circumstances.
The Varamatic system was also criticised for feeling too light at high speed, when greater weighting tends to give the driver more confidence in making fine steering adjustments. Later systems by other manufacturers would adjust the behaviour of the steering in reaction to road speed.