Making a modern automobile with its front wheel drive, wide tires and, in some cases, burdensome weight answer fingertip commands from a steering wheel takes a significant amount of effort.
A Little Bit of History
Most automotive historians consider Francis W. Davis to be the inventor of modern day power steering. He patented his "system for reducing the steering effort on vehicles by using an external power source" in 1932. Davis was originally a chief engineer for the truck division of the Pierce Arrow Motor Car Company, but held several positions at General Motors while further developing his patents.Chrysler Corporation introduced the first widely available power steering system, the Hydraguide, as an option on the 1951 Chrysler Imperial. The Hydraguide system was based on expired Davis patents for which he never received compensation. In 1952, Cadillac followed, introducing its own system derived from current Davis patents. As auto manufacturers continued using wider, low-pressure tires and moving engines forward onto the front axle, assisted steering became an increasingly available option.
Doing It All for the Driver
Power steering systems must transmit subtle driver commands to the front wheels dependably and with minimum distortion, regardless of power application and the position of the front suspension. They also must continually provide useful feedback through the steering wheel to the driver. Unfortunately, the customer generally notices most power steering systems only when something has gone wrong.The conditions power steering systems function in generally are harsh. Constant use, high temperatures and operating pressures upwards of 200 psi generate wear materials that degrade fluid performance, resulting in breakdown, loss of lubricating properties and varnish deposits. In addition, as power steering fluid circulates through the system, it picks up and distributes contamination from the breakdown of aging hoses and seals.
Pump It Up
The most common type of pump used in current power steering systems is the Rotary Vane. The pump works on the principle of positive displacement. Consisting of a series of vanes mounted to a rotor, the vanes provide circulation within the cavity by sliding in and out of the rotor during operation, creating a seal on the interior of the cavity. Liquid is captured in each chamber between the vanes and is forced through the system by the resulting rotation.Atmospheric pressure on the intake side of the pump helps to push fluid in, while pressure created by the rotating action moves the fluid out the discharge side of the pump. The pump is designed to provide an adequate flow of fluid when the engine is idling. It incorporates a pressure release valve to prevent system pressure from rising too high during high-rpm operation.
Hydraulic Fluid is Thicker Than Water
According to Patrick Burrow, technical director for International Lubricants Inc., manufacturers of LUBEGARD, "Power steering fluids are similar to fluids used to operate similar hydraulic systems. Power steering fluid is composed of base oil and an additive package. The additives help to reduce or eliminate foaming, prevent wear and oxidation and promote compatibility for the different types of seals, hoses and other chemically sensitive materials used in steering systems."While conventional base stocks often are used for reasons of economy, some manufacturers are specifying synthetic base oils, which have better performance characteristics but are more costly.
After the basic requirements of protection and compatibility are met, the most important function of the fluid is sufficient uniform viscosity. Because the fluid provides lubrication and hydraulic force, shear stability, or the fluid's ability to resist viscosity breakdown as it is mechanically worked between wearing surfaces, is an issue.
"Even though they may start thick, it doesn't take long for some base oils to shear down, providing at best very marginal system protection," says Burrow. So it is important to make sure the fluid used for the system will meet viscosity requirements.
Oxidation is one cause of decreased stability in power steering fluid. Once begun, a catalytic effect takes place, and these chemical reactions result in the formation of acids that cause corrosion and produce gum and varnish that increase wear and plug lines and valves. Heat, moisture, acids or solid contaminants accelerate the process of oxidation.
The mixing of air into the power steering fluid can cause aeration or foaming. Air usually enters the system through the hoses or seals. If fluid level in the reservoir is low, a vortex can develop, allowing air to enter the pump intake. Aeration in the system leads to non-lubricated hot-spots, which contribute to excessive pump wear. Aeration also increases the heat load on the hydraulic system.
Cavitation occurs when the volume of fluid demanded by the pump exceeds the volume of fluid being supplied. This normally occurs due to low fluid level in the reservoir. Vapor cavities form in the fluid and implode when compressed, generating heat and causing a characteristic knocking noise. The consequences of cavitation in a power steering system can be serious and include metal erosion, which damages hydraulic components and contaminates the fluid. In extreme cases, cavitation can cause mechanical failure of the pump.
Moisture contamination from high humidity or leaking seals is another cause of fluid breakdown. Moisture can form water droplets that saturate power steering fluid. The result is accelerated oxidation, foaming, overheating, rust and corrosion.
Pick a Fluid, Any Fluid
As power steering systems have become increasingly more complex, fluid needs have changed and so have the nature and types of lubricants specified. With the increased use of rack and pinion systems that use the same fluid as the power steering pump system, the problem of seal, hose and rubber component chemical deterioration has become an issue.
When choosing a fluid for their system, Assad Nassry, a formulator for Shell/Pennzoil says, "most manufacturers stay with Automatic Transmission Fluid (ATF) because it has gone through a lot of performance testing over the years." ATF has excellent hydraulic properties in addition to possessing anti-oxidant and anti-foaming properties. As long as the pump and system do not have a unique design, ATF is fine.
However, in some cases, the chemical makeup of ATF can cause certain materials used for seals to become soft, wear easily and start to leak.
"Some of the auto manufacturers switched from ATF to power steering fluid as seal and material compatibility dictated," says Burrow. Application-specific power steering fluids provide the desired seal and material compatibility with the physical properties to function well as a hydraulic fluid for the pump, while still protecting the steering components.
Recently, as manufacturers look for more efficiency, another issue has influenced the choice of fluids. Newer pumps are designed to operate more efficiently and enable the engineers to reduce fluid viscosity while providing adequate protection with a minimum of "parasitic" loss.
To Flush or Not to Flush
Fluid flushing is a fairly new concept for automotive manufacturers. For years there were very few criteria for replacing fluid other than observing the color and odor of the fluid and replacing it when it smelled burnt or changed color. With warranty costs rising, many manufacturers specify mileage intervals for preventive maintenance.
The new flushing systems that remove power steering fluid only from the fluid reservoir are reasonably effective at removing about 85 percent of the used fluid. If this fluid exchange is accompanied by the insertion of an (often magnetic) in-line filter on the low pressure return line, removal of dirt and wear metals will greatly extend the life of the system and components. While opinions vary on this type of service, if it is performed every 30,000 miles, the cost is reasonable and power steering systems and components will stay in excellent condition.
Maintenance is Key
Power steering systems often are the most neglected system on a vehicle and suffer from the use of improper or inferior quality fluids when the reservoir is refilled.
"The worst effect of using the wrong fluid type can be damage to seals, hoses and other system materials that can result in stuck valves, leaks or failure with resulting fluid loss," Burrow advises. If the additive package is weak or the fluid quickly degrades to too thin of a viscosity, protection from wear may be marginal.
The best way to prevent problems, says Burrow, is to pay attention to the quality and type of fluid used, making sure it is compatible with manufacturers' viscosity and material requirements. Other suggestions are to inspect the fluid level and condition on a regular basis. If problems are apparent, flush the system and install a high flow inline filter.
The aftermarket has responded to the need for manufacturer-specified fluids.
"You do not have to go to the dealership for the various 'specified' power steering fluids used in vehicles today," Burrow notes. "Products in the aftermarket are available that will meet the system needs for viscosity, performance and material compatibility."
With the aftermarket choices of quality lubricants offered, finding a fluid solution for your application should not be difficult and will allow you to meet or exceed the manufacturers' requirements.
Jim Marotta is a freelance writer with more than 17 years’ experience in automotive publishing. He was the associate editor of the Mobile Air Conditioning Society’s ACtion magazine and is currently a technical editor for ChiltonPRO.com.
