An Introduction To Power Steering Pump Types , Components , and Selection

An Introduction To  Power Steering Pump Types , Components ,  and Selection


Introduction

Power steering systems have contributed to reduced driver fatigue and made driving a  more pleasant experience. Nearly all power steering systems at the present time use fluid pressure to assist the driver in turning the front wheels. Since driver effort required to  turn the front wheels is reduced, driver fatigue is decreased. the advantages of power  steering have been made available on many vehicles, and safety has been maintained in these systems.

There are several different types of power steering systems, including integral, rack and  pinion, hydro boost, and linkage type. In any of these systems, the power steering pump is the  heart of the system because it supplies the necessary pressure to assist steering.

The power steering pump drive belt is a simple, but very important, component in the  power steering system. A power steering pump in perfect condition will not produce the  required pressure for steering assist if the drive belt is slipping.

The power steering pump, a part of your vehicle's power steering system, is the component that compresses the power steering fluid. It is basically a rotary vane pump that is powered by the engine via a belt and pulley. The construction of the pump includes a set of retractable vanes and an oval chamber.

How a Power Steering Pump works?

The power steering pump is used to put additional force onto the steering system when you turn the wheel. The pressure it provides comes from a rotary vane pump which is powered by the car’s engine. Inside this pump are retractable vanes that will spin inside an oval chamber and take power steering fluid from the lines at low pressure and distribute it at high pressure into the steering rack. As the pump is powered by the engine the speed in which the the pump flows is relative to the speed of the engine , meaning at high engine speeds the steering will operate quicker than an engine running at a lower speed. The pump has a pressure relief valve which will ensure the pressure isn’t too high , which is extremely effective on engines that are running at high speeds

FIG 1

Power Steering Pump Types

Different types of power steering pumps are used to power the system. The main difference between the different types of pumps is the design of the fins that move the steering fluid that is inside the pump and expelled through built-up pressure. They are all similar in that they contain a rotor inside the pump housing that spins. There are four different types of pumps used in power steering systems.

FIG 2

Vane Power Steering Pump

Vane pumps  (figure 3) are the most common type of power steering pump used. In this type of pump the rotor is housed in an oval- or elliptical-shaped housing where it turns. Vanes fitted to the outside diameter of the rotor sit against the housing walls as the rotor turns. When power steering fluid enters into the vane pump housing it is trapped between the vanes, the housing wall and the rotor. A subsequent pressure increase causes the fluid to be pumped out of the housing and then through the outlet chambers. 

FIG 3

Roller Power Steering Pump
In a roller power steering pump(figure 4), wide V-shaped grooves cut into the side of the rotor allow steel rollers to ride along the inside contour of the pump. The pump is contained in an oval-shaped housing within the pump body. Centrifugal force pushes the rollers to the oval's outer edge where they trap fluid, similar to the way the vanes catch the fluid in a vane pump. The pressurized fluid is forced out through two outlets in the pump, driving the power steering system.

FIG 4

Slipper Power Steering Pump
Like the vane and roller pump, the slipper power steering pump(figure 5) has a rotor housed in an elliptical-shaped chamber that rotates within the body of the pump. Fitted into wide slots on the rotor are springs that are topped with scrubber-type "slippers." The springs keep the slippers in constant contact with the wall of the pump. As fluid enters into the pump, pressure is built up and released to drive the power steering system.

FIG 5

Gear Power Steering Pump

In gear power steering pump (figure 6) the gears rotate they separate on the intake side of the pump, creating a void and suction which is filled by fluid. The fluid is carried by the gears to the discharge side of the pump, where the meshing of the gears displaces the fluid.

FIG 6

Useful Hints 

Despite the new advances, the majority of the vehicles on the road today still share a similar hydraulic powers steering system. Like any system in your vehicle, the first step to fixing its problems is understanding how it is supposed to work so you can see what is going wrong.

Your vehicle’s power steering system needs a few different components to work properly:
1- A good engine belt
2- A health power steering pump
3- Clear passageways
4- The proper level of clean power steering fluid
5- No leaks either in or out of the system
Failure Symptoms Of Power Steering Pump
The power steering pump directs fluid from the reservoir into the steering gear, which applies the correct amount of pressure to turn the wheels smoothly. There are several symptoms of a bad or failing power steering pump, so if you notice the following, have the pump inspected by a professional mechanic as soon as possible:

What are the Symptoms of Failure?
Steering system may leak
Lack of assist
Line restriction
Whining or squealing noise
Wheels are difficult to turn
Steering wheel vibrates while idling

1.Whining noise while turning the wheel
If you hear a whining noise while turning the wheel of your vehicle, something is wrong with your power steering system. It could be a leak in the power steering pump or the fluid level could be low. If the fluid level is left this way for too long it can damage the whole power steering system. Either way, the power steering pump needs to be looked at and potentially replaced by a professional.

2.Steering wheel slow to respond
While turning a corner, if your steering feels slow to respond to the steering wheel inputs you are making, chances are your power steering pump is failing. Along with this, you may also hear a whining noise. If you notice these two symptoms together, contact your technician to have your power steering pump replaced.

3.Stiff steering wheel
Not only can your steering wheel be slow to respond, it can also become stiff if the power steering is failing. If your steering wheel starts to feel stiff, your power steering pump may be going bad.

4.Squealing noises when the vehicle starts
While whining noises are usually heard when you turn the vehicle, squealing noises come when the vehicle first starts. They can happen when you make sharp turns as well, but they are more likely to happen for a minute as you start your car. The squealing noise will come from the hood of your vehicle, and is a sign your power steering pump may be going bad and causing the belt to slip.

5.Groaning noises
Groaning noises are the worst noises your power steering pump can make. They will get worse and worse as your power steering pump continues to fail. If the power steering system fails from lack of fluid it can damage the whole system including the steering rack and lines, and require complete replacement.

Power Steering Pump Design

Various types of power steering pumps have been used by car manufacturers. Many vane-type  power steering pumps have flat vanes that seal the pump rotor to the elliptical pump cam  ring (Figure 7). Other vane-type power steering pumps have rollers to seal the rotor to the  cam ring. In some pumps, inverted, U-shaped slippers are used for this purpose. the major 

FIG 7

differences in these pumps are in the rotor design and the method used to seal the pump rotor  in the elliptical pump ring. the operating principles of all three types of pumps are similar.

A balanced pulley is pressed on the steering pump drive shaft. this pulley and shaft are belt-driven by the engine. A spring-loaded lip seal at the front of the pump housing prevents 

fluid leaks between the pump shaft and the housing. the oblong pump reservoir is made from steel or plastic. A large O-ring seals the front of the reservoir to the pump housing (Figure 8 )

FIG 8

Smaller O-rings seal the bolt fittings on the back of the reservoir.the combination cap and dipstick keeps the fluid reserve in the pump and vents the reservoir to the atmosphere. Some power steering pumps have a variable assist steering actuator in the back of the pump housing. the PCM operates this actuator to provide increased steering assist at low vehicle speeds.

The rotating components inside the pump housing include the shaft and rotor with  the vanes mounted in the rotor slots. A seal between the output shaft and the housing prevents oil leaks around the shaft. As the pulley drives the pump shaft, the vanes rotate inside an elliptically shaped opening in the cam ring .the cam ring remains in a fixed position inside the pump housing. A pressure plate is installed in the housing behind the cam ring (Figure 9 )

FIG 9

A spring is positioned between the pressure plate and the end cover, and a retaining ring holds the end cover in the pump housing. the flow control valve is mounted in the pump housing, and a magnet is positioned on the pump housing to pick up metal filings rather than allowing them to circulate through the power steering system (Figure 10 ).

FIG 10

The flow control valve is a precision-fit valve controlled by spring pressure and fluid pressure. Any dirt or roughness on the valve results in erratic pump pressure. the flow control valve contains a pressure relief ball (Figure 11 ). 

High-pressure fluid is forced past the control valve to the outlet fitting. A high-pressure hose connects the outlet fitting to the inlet fitting on the steering gear. A low-pressure hose returns the fluid from the steering gear to the inlet fitting in the pump reservoir.

FIG 11

Typical Maintenance Kit

You have to know the parts that are included in a steering pump repair kit so you can compare kits from different suppliers and retailers. A typical repair kit has:
1- Seals

2- O-rings
3- Rollers, slippers, vanes (depending on the type)
4- Washers
5- Gaskets

POWER STEERING PUMP ASSEMBLY

1- Seal, Pump Body (figure 12)
2- Body, w/Seal, Pump
3- Seal, Pump Body
4- Seal, O-Ring
5- Shaft, w/Retaining Ring, Pump Drive
6- Plate, Thrust
7- Spring, Valve
8- Valve, Flow Control, Assy
9- Ring, Pump -Not Serviced Separately-
10- Ring, Shaft Retaining -Not Serviced Separately-
11- Rotor, Pump -Not Serviced Separately-
12- Plate, Pressure
13- Pin, Pump Dowel
14- Spring, Pressure Plate
15- Cap, Reservoir Filler Assy
16- Reservoir, Pump
17- Seal, O-Ring
18- Stud, Pump Mounting
19- Connector, Hose, Assy
20- Seal -Not Serviced Separately-
21- Seal -Not Serviced Separately-
22- Ring, End Cover Snap
23- Cover, End

FIG 12

Hydraulic modelling 

The hydraulic subsystem is based on the Wheatstone bridge representation seen in Figure 13.. 

FIG 13

It is assumed that opposite orifices in the bridge are equal in size. The model thus considers the flow equation for the four orifices and the continuity equation for each cylinder chamber and the volume between the pump and the valve. . The pump flow qs and load flow ql can be described by

where A1 = A[Tsw] and A2 = A[−Tsw].

 Tsw = Steering wheel torque [Nm]

 qs = System flow [m³/s]

 ql  = Load flow [m³/s]

 Cq = Flow coefficient

 PL = Load pressure [Pa]

 ps = System pressure [Pa]

 𝝆 = Density [kg/m³]

From these equations the load pressure pL and the pump pressure ps can be derived as 

where q = qL /qs

The load pressure is shown in Fig 14. for both a positive and a negative load flow, which describes the static characteristic of the valve. Finally, the opening areas can be derived as

FIG 14

To calculate the steering effort  requires inputs as listed :

1. Tyre Width (with reference to tyre designation). 

2. Front Axle Weight 

3. King Pin Off-set on Ground (scrub) 

4. Steering Arm Length 

5. Drop Arm Length 

6. Gear Box Ratio 

7. Steering Wheel Diameter 

Length of Steering Arm and Drop Arm are considered as 203 mm and 225 mm respectively with reference to N1 category vehicle. A steering box with 18:1 gear ratio is considered which can be power assisted and a 200 mm radius Steering Wheel is taken, which is generally ergonomic.

TSW = Total Kingpin Torque required to steer axle. 

W = Vehicle Weight supported by the steered axle. 

F = Coefficient of friction (dimensionless). 0.7 as a Maximum

B = Nominal width of the tire print (tread width). 

E = Kingpin Eccentric (use nominal tire width).

the pump has to deliver at least the flow amount that the hydraulic cylinder is demanding at required maximal speed

where :

qp = Flow delivered by the pump [m³/s]

x∙rmax = Rack speed  [m/s]

Ap = Cylinder area [m² ]