Module-3

POWER STEERING

Introduction:

The steering system of a vehicle is one of its key components. In a hydraulic power steering system, the effort required to turn the wheel of a vehicle by the rotation of the steering wheel is reduced with the help of hydraulic assistance. Power steering is an advanced form of steering system in which the overall effort required by the driver is reduced through an increase in the force applied on the steering wheel with the help of either electric or hydraulic assistance. In a normal steering mechanism, in comparison to the power steering mechanism, there is no hydraulic or electrical assistance to reduce the effort of turning the steering wheel. The rest of the mechanical components and their working remain the same between the two steering systems, minus the parts which are exclusive to a power steering system.

Three main types of power steering:

• Hydraulic Power Steering: In a hydraulic power steering system, the effort required to turn the wheel of a vehicle by the rotation of the steering wheel is reduced with the help of hydraulic assistance. When the steering wheel is turned, a hydraulic pump, which draws power from the vehicle’s engine, starts to pump hydraulic fluid through the system’s lines. This high-pressure hydraulic fluid then enters a cylinder and exerts force on the cylinder piston. This piston then pushes the hydraulic fluid ahead of it through the system’s lines, which in turn exerts pressure on the rack and pinion, coupling arrangement, multiplying the input force several times and resulting in the rotation of the vehicle’s front wheels.

• Electric Power Steering: In an electric power steering system, when the driver of a vehicle turns its steering wheel i.e., gives input, it is read by the sensors on the steering column and relayed to the Electronic Control Unit (ECU) of the vehicle. The ECU, after analysing these inputs, sends an electronic signal to an electronic motor. This motor is at the end of the steering column. The gears of this electric motor then provide torque on the basis of the received signals from the ECU to the pinion which relays this torque to the rack and hence, turns the wheels of the vehicle.

• Hybrid Power Steering: In a hybrid power steering system, the entire working concept remains the same as that of the hydraulic power steering system. The only significant difference is that the hydraulic pump, which is driven by the engine, is replaced with an electric pump. This makes the whole system a bit more reliable than the one in which a hydraulic pump is used.

Power steering systems normally use an engine-driven pump and hydraulic system to assist steering action. Pressure from the oil pump is used to operate a piston and cylinder assembly. When the control valve routes oil pressure into one end of the piston, the piston slides in its cylinders. Piston movement can then be used to help move the steering system components and front wheels of the vehicles.

The components that are common to all power steering systems are as follows:

POWER STEERING PUMP - The power steering pump is engine-driven and supplies hydraulic fluid under pressure to the other components in the system. There are four basic types of power steering pumps - vane, roller, slipper, and gear types. A belt running from the engine crankshaft pulley normally powers the pump. During pump operation, the drive belt turns the pump shaft and pumping elements. Oil is pulled into one side of the pump by vacuum. The oil is then trapped and squeezed into a smaller area inside the pump. This action pressurizes the oil at the output, as it flows to the rest of the system. A pressure relief/flow valve is built into the pump to control maximum oil pressure. This action prevents system damage by limiting pressure developed throughout the different engine speeds.

CONTROL VALVE - The control valve (rotary or spool type), which is actuated by steering wheel movements, is designed to direct the hydraulic fluid under pressure to the proper location in the steering system. The control valve may be mounted either in the steering mechanism or on the steering linkage. depending on which system configuration is used. POWER STEERING HOSES - Power steering hoses are high-pressure. hydraulic rubber hoses that connect the power steering pump and the integral gearbox or power cylinder. One line serves as a supply line, the other acts as a return line to the reservoir of the power steering pump.

There are three major types of power steering systems used on modern passenger vehicles - integral piston (linkage type), external cylinder (linkage type), and rack and pinion. The rack and pinion can further be divided into integral and external power piston. The integral rack and pinion steering system is the most common.

Integral Piston Power steering (Linkage Type)

The integral piston (linkage type) power steering system has the hydraulic piston mounted inside the steering gearbox. This is the most common type of power steering system. Basically, this system consists of a power steering pump, hydraulic lines, and a special integral power-assist gearbox.

The integral piston power steering gearbox contains a conventional worm and sector gear arrangement, a hydraulic piston, and a control valve. The control valve may be either a spool valve or a rotary valve depending upon manufacturer.

The operation of an integral power steering system is as follows:

With the steering wheel held straight ahead or in NEUTRAL position, the control valve balances hydraulic pressure on both sides of the power piston. Oil returns to the pump reservoir from the control valve.

For a right turn, the control valve routes oil to the left side of the power piston. The piston is pushed to the right in the cylinder to aid pitman shaft rotation.

For a left turn, the control valve routes oil to the right side of the power piston. The piston is pushed to the left in the cylinder to aid pitman shaft rotation.

In both left and right turns piston movement forces oil on the non-pressurized side of the piston back through the control valve and to the pump. Semi Integral power steering (External Cylinder Linkage Type)

The external cylinder power steering system has the power cylinder mounted to the frame and the center link. In this system the control valve may be located in the gearbox or on the steering linkage. Operation of this system is similar to the one previously described.

Power Rack and Pinion

Power rack-and-pinion steering uses hydraulic pump pressure to assist the operator in moving the rack and front wheels. A basic power rack-and-pinion assembly consists of the following:

POWER CYLINDER (hydraulic cylinder formed around the rack) - The power cylinder is precisely machined to accept the power piston. Provisions are made for the hydraulic lines. The power cylinder bolts to the vehicle frame, just like the rack of a manual unit.

POWER PISTON (a double-acting hydraulic piston formed on the rack) - The power piston is formed by attaching a hydraulic piston to the center of the rack. A rubber seal fits around the piston to prevent fluid from leaking from one side of the power cylinder to the other.

HYDRAULIC LINES (steel tubing that connects the control valve and power cylinder).

CONTROL VALVE (a hydraulic valve which regulates hydraulic pressure entering each end of the power piston) - There are two types of control valves - rotary and spool. Using a torsion bar connected to the pinion gear operates the rotary valve, whereas the spool valve is operated by the thrust action of the pinion shaft.

Other components of the power rack and pinion are similar to those that are found on manual rack-and-pinion steering system.

Power rack-and-pinion operation is fairly simple. When the steering wheel is turned, the weight of the vehicle causes the front tire to resist turning. This resistance twists a torsion bar (rotary valve) or thrusts the pinion shaft (spool valve) slightly. This action moves the control valve and aligns the specific oil passages. Pump pressure is then allowed to flow through the control valve, the hydraulic line, and into the power cylinder. Hydraulic pressure then acts on the power piston and the piston action assists in pushing the rack and front wheels for turning.

Steering of tracked vehicle:

Skid-steer Drive

Skid-steer locomotion is commonly used on tracked vehicles such as tanks and bulldozers, but is also used on some four- and six-wheeled vehicles. On these vehicles, the wheels (or tracks) on each side can be driven at various speeds in forward and reverse (all wheels on a side are driven at the same rate). There is no explicit steering mechanism--as the name implies steering is accomplished by actuating each side at a different rate or in a different direction, causing the wheels or tracks to slip, or skid, on the ground.

In the above left figure, the wheels on the left side are driven forward and the wheels on the right side are driven in reverse at the same rate. The result is a clockwise zero radius turn about the center of the vehicle shown in the right figure. Note that throughout the turn the wheels are required to skid on the ground, with the front and rear pair of wheels skidding more than the center pair. Skidding has some disadvantages including tire/track wear but for tracked vehicles there is no alternative. (Vehicles that use skid-steer usually are off-road types such as construction equipment and tanks--the reduced friction of a non-paved surface helps to reduce tire/track wear.)

Skid-steer is closely related to the differential drive system, replacing the caster wheel with extra drive wheels. It has the same disadvantage: moving in a straight line requires the wheels on each side to be turning at the same speed, which can be difficult to achieve. The advantage of skid-steer is increased traction and no "caster wheel effect".

The wheels on a skid steer typically have no steering mechanism, they are in a fixed, straight line relative to the body of the machine. By turning the left and right wheel pairs at different speeds, the machine turns by skidding, or dragging its wheels across the ground. The rigid frame and strong wheel bearings prevent the torsional forces caused by this dragging motion from damaging the machine. This skidding motion tears up the ground on which the machine operates.

Articulated steering

Hydraulic power is used to turn a whole section ofa machine on a vertical hinge. This design is called articulated steering and it is controlled by a steeringwheel, a hydraulic control valve, and hydraulic cylinders. (See fig. 3-3.) The pivot is midway in thevehicle, so both parts share equally in the pivoting. Thisaction produces the effect of four-wheel coordinated steering, such as the front-and-rear wheels run in eachothers tracks, backward and forward. Conventional tractors with king-pin steering turn the front wheels only, which then run outside the tracks of the rear wheels. A larger turning radius is required to ensure sufficient clearance for the rear wheels. On an articulated Holder tractor, the complete front end turns into the steering direction. Front and rear wheels always run in the same track. Attachments can be moved even at standstill. Inching attachments into position becomes easy.

FRONT-AND-REAR STEERING

Wheeled equipment may be designed to steer by angling the front wheels, the rear wheels, and or both these problems must be documented and turned in the front-and-rear wheels (fig. 3-4). Front-wheel for repairs. Steering is the standard method. The vehicle follows the

Angling of the wheels and the rear wheels do not go behind the bucket on turns and keeps the front tires outside the path of the front ones, but trail inside. Tracking in the rear while backing away from banks and Rear-wheel steering swings the rear wheels outside dump trucks. In new equipment, this design has been of the front-wheel tracks. The principal advantage is replaced by articulation. greater effectiveness in handling off-centre loads at In four-wheel steering, the front wheels are turned either the front or rear and preventing path down a one way and the rear wheels are turned to the same angle side slope. This type of steering is used with front-end in the opposite direction. The trailing wheel always loaders, as it keeps the weight of the machine squarely moves in the same track as the leading wheel whether the equipment is moving forward or backward. This design lessens rolling resistance in soft ground, because one set of tires prepares a path for the other set. Additionally, this design provides maximum control of the direction of the load. Also, it enables the equipment to be held on a straight course and permits short turns in proportion to the maximum angle of the wheels. In crab steering, both sets of wheels are turned in the same direction. If both sets of wheels are turned at the same angle, the machine moves in a straight line at an angle to its centre line. Results can be obtained from either four-wheel steering or crab steering by using different turning angles on independently controlled front-and-rear wheels.

Clutch brake steering control

Clutch-brake steering systems are known to operate on track vehicles which are highly reliable and cost effective, but lacked the fine degree of controllability found in the more expensive differential steering. An electronically controlled clutch-brake steering control apparatus provides control signals to electro hydraulic proportional valves for accurately modulating the control pressures of a set of steering clutches and brakes. A pair of sensors delivers signals, which are indicative of track speed, to the control apparatus. The apparatus acts to adaptively adjust selected ones of the valves to maintain a desired track speed differential. Thus, the controllability of the clutch-brake steering system is enhanced to effectively compete with more expensive and complex systems. Tracked Vehicle Steering

In order to steer a tracked vehicle, it is necessary to drive one track faster than the other, causing the vehicle to turn toward the slower track. This is called "skid steering" or "differential steering". While the theory is simple, it’s execution is not.

Design Considerations

A steering transmission must, in addition to steering the vehicle, be easy to use. Most fast track-layers are tanks: incredibly heavy, powerful, and expensive machines that are operated by teenage recruits with limited experience, at night without lights, over rough and unfamiliar ground, with extremely limited vision… not to mention under fire. Thus, whatever steering mechanism is used, it has to be fairly simple to operate. It must also be efficient. Any inefficiency produces waste heat, usually from the friction in a slipping clutch or brake. Since a tracked vehicle implies a heavy vehicle, which in turn implies a powerful engine, inefficiency can produce problematic amounts of waste heat. For example, a 1500 hp engine running a 50% efficient transmission will produce 560,000 watts of waste heat. All this heat has to be extracted from the vehicle before it causes problems. Furthermore, all that energy is no longer available for driving the vehicle, meaning the engine ... and fuel tank ... could be made much smaller if the transmission were more efficient.

Both of these problems are significantly less for slow track-layers, such a bulldozers, but they still apply.

Drive Systems

Dual Drive

The simplest way to achieve this is to drive each track with a separate power source. The earliest tracked vehicles, including the Holt Tractor of the late 1800’s and the Whippet, a WW I British Light Tank, operated on this principle. However, it has its drawbacks.

First of all, it requires two engines. In the case of a steam powered tractor, this isn’t such a problem, since the "engine" is only a small part of the whole, and two (or more) can, and often do, share a single boiler. In the case of internal combustion engines, however, it requires two complete engines, with all the attendant extra weight, complexity, and maintenance headaches. In this case, two engines does not result in twice the reliability, but half the reliability, as if either engine fails, the vehicle is effectively immobilized and capable only of spinning in circles.

A second problem is that it becomes difficult to drive in a straight line. Track speed is a function of power and ground drag ... while it is possible to coordinate two engines such that they produce the same amount of power, it is highly unlikely that each track will experience the same amount of drag, and as a result the tracks will turn at slightly different speeds. Moreover, the drag on each track will be constantly changing, making continual adjustments necessary.

However, driving each track with a separate power source has one advantage ... by reversing one engine, it is possible for the vehicle to turn in place, called a "pirouette" or a "neutral turn". The advantages in maneuverability are obvious.

The difficulty in traveling in a straight line is minor at low speed, so modern slow track- layers like bulldozers often use this system, though slightly modified ... with a single engine driving a hydraulic pump, and hydraulic motors driving each track. Many remote controlled tracked toys use separate electric motors, as this system is very cheap and simple to build.

Clutch-Brake Steering

Far less complicated (as it only requires one engine) is the Clutch-Brake system, where the output of a single power source drives both tracks directly. Since they are physically connected to each other, the tracks must turn at the same speed and the vehicle will travel in a straight line. To allow for turns, each track can be disconnected from the engine with a clutch, allowing that track to slow and the vehicle to turn fairly gently ... a "free turn". A brake allows the disengaged track to be slowed to tighten the turn, even to the point of stopping the track so the vehicle turns in a very tight radius ... a "braked turn".

This system is fairly simple and easy to drive, and most of the tanks of the First World War used this method of steering. However, it is not very efficient. Braking one track slows the vehicle, and wastes a large portion of the power produced by the engine to be converted into heat. While this is not a significant problem in a small vehicle, a large vehicle with a large engine can produce a tremendous amount of heat in a very short time. Braking one track also slows the vehicle down significantly, which is a consideration in military vehicles where speed is paramount.

It is also a bit unpredictable in steering. The braking force vs. yaw curve is basically flat, meaning that a tiny change in braking force can result in a huge change in the rate of turn. If the clutch to one track is disengaged while climbing a very steep slope, the turn it produces may be quite dramatic even without the application of the brake. While descending a very steep slope, the turn may even be in the wrong direction!

Lastly, the Clutch-Brake system does not allow a vehicle to execute a neutral turn.

Geared Steering

One solution that solved both the inefficiency problem and the unpredictability in steering problem is that of Geared Steering. In this design, a single engine is connected to the tracks through separate transmissions, and the vehicle is steered by selecting different gears for each track. For example, driving the left track in 1st gear and the right track in 2nd gear would result in the left track turning more slowly and a left turn. If the gear ratios are not even multiples, a great number of turning radii can be achieved, and if each transmission includes a reverse gear, then a neutral turn can be accomplished.

This system requires great driver skill, but is efficient and not terribly complex. One minor drawback is that the vehicle cannot be steered while in 1st gear.

Braked Differential Steering

The Clutch-Brake Drive can be simplified somewhat by driving the two tracks through a differential and eliminating the clutches. The result is the Braked Differential, wherein as one track was slowed, the differential gears rotate and speed up the other track.

This system is even less efficient than the Clutch-Brake system, in that the brake not only dissipates the energy of the slowed track, but also part of the power of the engine. It also leads back to the problem of straight-line travel, and the system was even less predictable in steering. However, it is extremely simple, and can be based around a conventional automotive rear axle and brakes.

Again, the braked differential system does not allow a neutral turn. "Cletrac", or Controlled Differential Steering

The "Clectrac" system was introduced by the Cleveland Tractor Company in 1921. In this system, a differential is designed such that by applying a brake, the differential can be forced to rotate at a given rate in proportion to speed. This system produces a turn of a single fixed radius, but is quite efficient. By slipping the brake, turns of greater radius can be achieved, but the efficiency advantage is lost.

This system could, like the Clutch-Brake or Braked Differential systems, "self-steer". However, as the brake force vs. yaw curve was fairly direct, it was easy to control.

This system was widely used in military vehicles during WW II.

"Maybach" Double Differential Steering

The Maybach system is a refinement of the Clectrac system, wherein instead of the differential gears precessing around a fixed gear, the differential gears precess around a gear driven at a speed proportional to engine speed. The result is a different radius turn for each forward gear.

This system is also capable of neutral turns, by leaving the main gearbox in neutral (hence the name "neutral turn"), and rotating the differential gears.

This system was used by the Panther, often considered the best tank of WW II. It had seven forward gears, and thus seven turning radii, not including a neutral turn.

Double Differential Steering

A further refinement of the Maybach system, developed by Major Wilson of the UK in 1928. This involves adding a second transmission between the steering input of the transmission and the engine. This allows the differential gears to be driven at several different rates in proportion to engine speed, and thus provides a multitude of turning radii, each of which is in turn proportional to the selected forward gear.

Furthermore, it is not subtractive like the Maybach transmission, as while it slows one track, it speeds up the other track.

This is essentially the forerunner of all modern fast track-layer steering transmissions, as it has all the essential characteristics: it is efficient, will not self-steer, allows multiple turning radii, and allows a neutral turn.

Using a hydraulic motor or electric motor to drive the steering shaft, instead of a fixed ratio transmission, allows for any turning radius at all, including zero (a neutral turn). While this is an improvement in some ways, it is a loss in others, due to the modest efficiency of electric and hydraulic systems. This often leads designers to prefer a limited number of turning radii rather than having to design for the waste heat generated.

I've created an animation of this transmission (AVI format, 16 seconds, 7 meg). It shows the vehicle traveling straight (both output shafts turning at 3.6 degrees per frame), turning right (left 6.0°/f, right 1.2°/f), turning left (same but opposite), and performing a left neutral turn (left -2.4°/f, right +2.4°/f). Watch the input and output shafts carefully. This design uses normal differentials instead of the planetary differentials of the diagram above. Triple Differential Steering

Adding a third differential produces a transmission that is, in a sense, a double differential transmission with a braked differential transmission for steering.

Virtually all modern fast track-layers use triple differential steering.

Here is an animation of this transmission (AVI format, 8 seconds, 4 meg) which shows the vehicle traveling straight (both output shafts rotating at 2.8°/f) and turning left (left 2.0°/f, right 3.6°/f). Again, it uses normal differentials instead of the planetary differentials used in the diagram.

I am not, however, sure I fully understand this transmission. My research indicates that both input shafts are driven by the engine, but I can't for the life of me see how it can perform a neutral turn unless the input shafts are driven independantly. I cannot conceive that modern tanks would forgo the advantage of being able to perform a neutral turn.

Planetary Steering

Some tracked equipment may be steered by a system that combines planetary steering and pivot brakes. The planetary steering system (fig. 6-10) differs from the one previously described in that the planetary pinion gears are two gears of different sizes, machined into one piece. Two sun gears are also included. One sun gear is splined to the sprocket pinion shaft, and the other is machined on the steering brake hub. The sun gear, machined to the steering brake hub, performs the same function as the ring gear in a conventional planetary system. Bushings are used to isolate the sprocket drive shafts and the steering brake hubs from the bevel gear carrier and the planetary carrier. Lubrication is provided from the oil sump located below the assembly.

When the tracked equipment travels straight ahead, its steering brakes are held in the applied position by heavy coil springs. Braking prevents the steering brake hub and sun gear from rotating and forces the large planetary pinion gears to "walk" around the sun gear. Then power is transmitted to the sun gear on the sprocket drive shaft from the smaller planetary pinion gears.

When a gradual turn is being made, the operator moves one of the steering levers back far enough to release the steering brake on one end of the planetary system. When the brake is released, the planetary pinion gears stop "walking" around the sun gear on the steering brake hub. This hub then rotates with the planetary carrier, and no power is transmitted to the sprocket drive shaft.

Occasionally, an adjustment of the steering brake is required to prevent slippage when it is engaged. Consult the manufacturer's service manual for adjustment procedures.

Pivot Brakes

The pivot brakes on tracked equipment are of the multiple disc type. Pulling the steering brake levers fully to the rear operates them. The middle discs (splined to the sprocket drive shaft) have laminated linings. The intermediate discs (held in position by studs) are smooth steel discs. An actuating disc assembly is two steel plates with steel balls between them. The assembly is located in the center of the discs and is connected to the operating linkage.

Ramps are machined on the steel plate of the actuating disc assembly, so when the brakes are applied, the steel balls move up the ramps and force the plates apart. Movement of the plates causes the discs to be squeezed together and to stop rotation of the sprocket drive shaft. When these brakes are fully applied, the tracks will stop. The steering levers are linked to the brakes independently to actuate them for sharp turns.

Adjustment of the pivot brakes is required to provide adequate braking with the steering levers. An adjustment is required when the steering levers can be pulled against the seat with the engine running. Consult the manufacturer's service manual for proper adjustment procedures.