Hydraulic cylinders are actuation devices that utilize pressurized hydraulic fluid to produce linear motion and force. Hydraulic cylinders are used in a variety of power transfer applications. Operating specifications, configuration or mounting, materials of construction, and features are all important parameters to consider when searching for hydraulic cylinders.

Important operating specifications for hydraulic cylinders include the cylinder type, stroke, maximum operating pressure, bore diameter, and rod diameter. Choices for cylinder type include tie-rod, welded, and ram. A tie-rod cylinder is a hydraulic cylinder that uses one or more tie-rods to provide additional stability. Tie-rods are typically installed on the outside diameter of the cylinder housing. In many applications, the cylinder tie-rod bears the majority of the applied load. A welded cylinder is a smooth hydraulic cylinder that uses a heavy-duty welded cylinder housing to provide stability. A ram cylinder is a type of hydraulic cylinder that acts as a ram. A hydraulic ram is a device in which the cross-sectional area of the piston rod is more than one-half the cross-sectional area of the moving component. Hydraulic rams are primarily used to push rather than pull, and are most commonly used in high pressure applications. Stroke is the distance that the piston travels through the cylinder. Hydraulic cylinders can have a variety of stroke lengths, from fractions of an inch to many feet. The maximum operating pressure is the maximum working pressure the cylinder can sustain. The bore diameter refers to the diameter at the cylinder bore. The rod diameter refers to the diameter of the rod or piston used in the cylinder.

Recirculating-ball Steering

Recirculating-ball steering is used on many trucks and SUVs today. The linkage that turns the wheels is slightly different than on a rack-and-pinion system.





The recirculating-ball steering gear contains a worm gear. You can image the gear in two parts. The first part is a block of metal with a threaded hole in it. This block has gear teeth cut into the outside of it, which engage a gear that moves the pitman arm (see diagram above). The steering wheel connects to a threaded rod, similar to a bolt, that sticks into the hole in the block. When the steering wheel turns, it turns the bolt. Instead of twisting further into the block the way a regular bolt would, this bolt is held fixed so that when it spins, it moves the block, which moves the gear that turns the wheels.

Instead of the bolt directly engaging the threads in the block, all of the threads are filled with ball bearings that recirculate through the gear as it turns. The balls actually serve two purposes: First, they reduce friction and wear in the gear; second, they reduce slop in the gear. Slop would be felt when you change the direction of the steering wheel -- without the balls in the steering gear, the teeth would come out of contact with each other for a moment, making the steering wheel feel loose.

Power steering in a recirculating-ball system works similarly to a rack-and-pinion system. Assist is provided by supplying higher-pressure fluid to one side of the block.

Now let's take a look at the other components that make up a power-steering system.

The Basic Idea

The basic idea behind any hydraulic system is very simple: Force that is applied at one point is transmitted to another point using an incompressible fluid. The fluid is almost always an oil of some sort. The force is almost always multiplied in the process.

in this drawing, two pistons (red) fit into two glass cylinders filled with oil (light blue) and connected to one another with an oil-filled pipe. If you apply a downward force to one piston (the left one in this drawing), then the force is transmitted to the second piston through the oil in the pipe. Since oil is incompressible, the efficiency is very good -- almost all of the applied force appears at the second piston. The great thing about hydraulic systems is that the pipe connecting the two cylinders can be any length and shape, allowing it to snake through all sorts of things separating the two pistons. The pipe can also fork, so that one master cylinder can drive more than one slave cylinder if desired. The neat thing about hydraulic systems is that it is very easy to add force multiplication (or division) to the system. If you have read How a Block and Tackle Works or How Gears Work, then you know that trading force for distance is very common in mechanical systems. In a hydraulic system, all you do is change the size of one piston and cylinder relative to the other

Hydraulic multiplication. The piston on the right has a surface area nine times greater than the piston on the left. When force is applied to the left piston, it will move nine units for every one unit that the right piston moves, and the force is multiplied by nine on the right-hand piston. Click the red arrow to see the animation.

To determine the multiplication factor, start by looking at the size of the pistons. Assume that the piston on the left is 2 inches in diameter (1-inch radius), while the piston on the right is 6 inches in diameter (3-inch radius). The area of the two pistons is Pi * r². The area of the left piston is therefore 3.14, while the area of the piston on the right is 28.26. The piston on the right is 9 times larger than the piston on the left. What that means is that any force applied to the left-hand piston will appear 9 times greater on the right-hand piston. So if you apply a 100-pound downward force to the left piston, a 900-pound upward force will appear on the right. The only catch is that you will have to depress the left piston 9 inches to raise the right piston 1 inch.

The brakes in your car are a good example of a basic piston-driven hydraulic system. When you depress the brake pedal in your car, it is pushing on the piston in the brake's master cylinder. Four slave pistons, one at each wheel, actuate to press the brake pads against the brake rotor to stop the car. (Actually, in almost all cars on the road today two master cylinders are driving two slave cylinders each. That way if one of the master cylinders has a problem or springs a leak, you can still stop the car.)

In most other hydraulic systems, hydraulic cylinders and pistons are connected through valves to a pump supplying high-pressure oil. You'll learn about these systems in the following sections.

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Hydraulic cylinders get their power from pressurized hydraulic fluid, which is typically oil. The cylinder consists of a cylinder barrel, in which a piston connected to a piston rod is moving. The barrel is closed by the cylinder bottom and by the cylinder head where the piston rod comes out of the cylinder. The piston has sliding rings and seals. The piston divides the inside of the cylinder in two chambers, the bottom chamber and the piston rod side chamber. The hydraulic pressure acts on the piston to do linear work.

A hydraulic cylinder is the actuator or "motor" side of this system. The "generator" side of the hydraulic system is the hydraulic pump, that brings a fixed or regulated flow of oil into the system. Mounting brackets or clevisses are mounted to the cylinder bottom as well as the piston rod.

By pumping hydraulic oil to the bottom side of the hydraulic cylinder, the piston rod starts moving upward. The piston pushes the oil in the other chamber back to the reservoir. If we assume that the oil pressure in the piston rod chamber is zero, the force on the piston rod equals the pressure in the cylinder times the piston area. If the oil is pumped into the piston rod side chamber and the oil from the piston area flows back to the reservoir without pressure, the pressure in the piston rod area chamber is Pull Force/(piston area - piston rod area). In this way the hydraulic cylinder can both push and pull.

SERVICING RECIRCULATING BALL STEERING GEARS, Larry Carley, Brake & Front End, January 2002

Though most vehicles today have rack & pinion steering, some still use a recirculating ball steering gear. The recirculating ball design, which dates back to the 1950s, was used on many rear-wheel drive cars up through the 1980s, as well as most pickup trucks. It’s still used today on a handful of vehicles, including some RWD cars and trucks.

The main advantages of the recirculating ball steering gear are its compact design and low friction characteristics. The manual version takes much less driver effort to steer the vehicle than a manual rack, making it well-suited for larger, heavier vehicles. But the design doesn’t lend itself very well to front-wheel drive applications because it uses a parallelogram steering linkage with a pitman arm and idler arm. For FWD, rack & pinion is the better choice.

HOW IT WORKS
In a manual recirculating ball steering gear, the steering column input shaft connects to a worm gear inside the box. The worm gear acts like a screw and moves a ball nut back and forth. The ball nut has teeth on one side that rotates a sector gear attached to the pitman arm. So when the driver turns the steering wheel, the worm gear inside the box slides the ball nut one way or the other. This rotates the sector gear which moves the pitman arm to work the steering linkage and turn the front wheels.

Ball bearings are used to reduce friction between the ball nut and worm gear. The balls roll in a series of grooves between the ball nut and worm gear. Each ball pushes the next one along until they reach the end of the path. A return tube then carries the balls back around the outside of the ball nut so they can reenter the grooves from the opposite end. This creates a continuous loop of balls as the worm gear turns. That’s why they call it a recirculating ball steering gear.

To further reduce friction, manual steering boxes are filled with a light viscosity grease (gear oil or ATF should NOT be used). The lubricant requires seals on the input shaft and pitman arm shaft. These seals are prone to leak after many years of service, which causes a loss of lubricant and accelerates wear inside the steering box. Overfilling the steering box with grease (it should only be about 2/3 full) may create too much pressure inside the box and force out the seals.

CRITICAL ADJUSTMENTS
The worm and sector gears have bearings as well as thrust washers or spacers that are used to adjust internal clearances. The input shaft and sector gear output shaft also have adjustment plugs, screws or shims for adjusting worm bearing preload and gear mesh preload. Accurate setting of these critical adjustments is essential because excessive clearances can make the steering feel loose, while insufficient clearances may cause the steering to bind or wear prematurely.

Worm bearing preload is a measure of how much force is required to turn the steering gear input shaft, which depends on the amount of preload and clearance on the worm gear thrust bearings. The tighter the fit, the more force it takes to rotate the shaft.

The preload is set by turning the large adjuster plug at the end of the worm shaft and then measuring the amount of effort it takes to rotate the shaft using an inch-pound torque wrench. Specifications vary depending on the application, so don’t guess if you don’t have accurate information or have never done this before. Adjustments are best left to those who have had experience rebuilding steering gears.

Gear mesh preload is the amount of effort required to rotate the sector gear shaft, which depends on the clearance (lash) between the ball nut and sector gear. The adjustment is made by turning an adjuster screw or bolt on top of the sector shaft. Like worm bearing preload, it is also measured with an inch-pound torque wrench. But this measurement is typically made with the steering in the center position because this is where steering feel is most noticeable. Gear mesh preload has the greatest effect on steering feel and play. Too much lash will reduce preload and make the steering feel loose. Not enough lash will increase preload and may cause the gears to bind — especially if the vehicle has a lot of miles on it and the steering gears are worn.

One of the most common mistakes that’s made on recirculating ball steering systems is overtightening the sector shaft adjuster screw in an attempt to eliminate looseness or play in the steering. The adjuster screw is usually easy to reach, so it’s the first thing that’s tightened down. But a small adjustment goes a long way here, so the screw should not be turned more than about 1/8th turn before rechecking steering effort and play.

Before any adjustments are attempted, however, the first thing that should be done is to thoroughly inspect the steering linkage, suspension and wheel bearings to see if any parts are worn or damaged. This includes inner and outer tie rod ends, the center link, idler arm, pitman arm and wheel bearings. If no worn parts are found, the steering box is probably suffering from center wear — which will likely take more than a simple adjustment to fix. It will require rebuilding or replacing the steering gear.

POWER STEERING
Power-assisted recirculating ball steering gears work the same way as manual gears, except that they have a sliding spool valve or a rotating spool valve and torsion bar to route hydraulic pressure to chambers on either side of a power piston that may be located inside or outside the steering box. With the internal assist type, the power piston is part of the ball nut assembly and the hydraulic chambers are on either side of the piston inside the steering box. With the external assist type, hose connect the steering box to a hydraulic cylinder attached to the steering linkage.

The amount of power assist or "reaction control" is determined by the diameter of the torsion bar. Vehicle manufacturers fine tune the steering feel to a particular model of vehicle and powertrain by using different sized torsion bars. Thus, a larger, heavier luxury car would typically have more built-in steering assist than a smaller, sportier car. Yet the steering boxes on both cars might appear to be identical from the outside.

AN EXPERT OPINION
Bill Mullins of Mullins Steering Gears in Lake Havasu, AZ, is one of the few people who specialize in rebuilding steering gears. He says if a recirculating ball steering gear is worn or is suffering from an internal problem, the safest approach for most technicians today is to replace the steering box rather than attempting to rebuild it.

"Unless you have specific training on how to rebuild steering gears, it’s best not to attempt it yourself for a number of reasons. One is that most people don’t know what to measure or the proper procedures for setting preload adjustments. It takes experience, the right tools and access to the manufacturer’s specifications. Also, internal components are not readily available from parts stores or dealers, so if internal parts are worn out there’s no way to rebuild the box unless you can get new parts. Even for a professional rebuilder, it’s often difficult to obtain certain parts. It all depends on the application. For some units you can get bushings and other parts, but for others you can’t."

Mullins said it’s also important to make sure a replacement gear is the same as the original. "When a technician calls his local parts store for a rebuilt steering gear, he may end up with a generic gear that looks the same on the outside, but is different inside. It may have a different steering ratio, or shorter or longer steering stops. The gear will fit, but will alter the way the steering feels, the way the car handles or its U-turn radius."

"Take a ‘92 Camaro, for example. GM offered a number of different steering boxes on the same model year depending on the engine and tire size on the vehicle," commented Mullins. A steering box for a Z28 had an entirely different ratio, feel and stops than one for a six-cylinder Camaro. So if you install the wrong replacement gear, your customer may not be happy with the results.

"The other alternative to simply replacing the steering gear is to send the original unit to a specialty rebuilder, like ourselves, who will go through it and recondition it to like-new condition," commented Mullins.

Mullins said that over the years Ford and Chrysler have used steering gears that appear to be the same on the outside, but provide different amounts of power assist and road feel by using different spool valve springs and torsion bars internally. "They may have dozens of variations of the same basic Saginaw model 800 steering gear, but you can’t tell one from the other just by looking at it. It’s what inside that counts."

STEERING GEAR REPLACEMENT
One thing technicians should always do before replacing a steering gear is to count how many revolutions of the steering wheel it takes to go from lock to lock. This will tell you if a vehicle has a standard steering ratio or a quick ratio. A standard gear ratio of 18:1 might take 3-1/2 turns lock to lock, while a quick gear ratio of 13:1 might only take 2-1/2 turns lock to lock.

For proper identification, it may be necessary to also have the vehicle VIN number and engine displacement as well as the year, make and model to order the correct replacement gear.

Removing a recirculating ball steering gear is usually a straight forward process. Before unbolting anything, make sure the steering wheel is centered with the front wheels aimed straight ahead. The input shaft coupling can then be disconnected from the steering column shaft, and the pitman arm removed or disconnected from the steering linkage. The gearbox itself is usually bolted to the frame with three or four bolts. Installation is just the reverse of the removal procedure. Just make sure the steering gear is in the center position before reattaching the pitman arm and steering column shaft.

With power units, all the old power steering fluid should be drained and the pump flushed to remove any possible contaminants before the new gear is installed. Also, hose should be carefully inspected and replaced as needed if they show any signs of deterioration or leakage. Hose can deteriorate internally allowing small pieces of rubber to flake loose and lodge in the valves within the pump or steering gear. This can cause the pump to work harder in an attempt to overcome the blockage, resulting in increased noise and steering effort. That’s why most experts recommend replacing old hose to prevent expensive pump and/or steering gear damage and comebacks.

When hose are replaced, the system should always be flushed to remove all the old fluid before the hose are attached to the new steering gear. Power steering fluid breaks down as it ages and accumulates contaminants which can damage a new gear or pump. The viscosity of the fluid also increases over time which can increase steering effort when the fluid is cold. The fluid can also leave varnish deposits that can affect the operation of the pump and spool valves in the rack.

Another level or protection can be added to the system by installing an aftermarket inline fluid filter. The filter will trap any residual debris in the system and provide ongoing protection as the system ages.

When the system is refilled with fluid, it will have to be purged of air. This is done by starting the engine, allowing the power steering fluid to warm up, then slowly turning the steering wheel from side to side, six to 10 times (do not turn too rapidly, do not hold at either locked position or allow the pump reservoir to run dry). If the fluid appears tan, light red, foamy or cloudy, it’s still full of air. Continue purging until the fluid clears. Make sure the fluid reservoir is full when you’re finished.

Always use the type of power steering fluid specified by the vehicle manufacturer. Do not use ordinary ATF unless it is specified by the vehicle manufacturer. Using the wrong type of fluid may cause seal swelling and leakage and may void the warranty on a replacement pump or steering gear.

Another item that needs to be inspected, adjusted and possibly replaced is the drive belt for the power steering pump. Replace the belt if it is frayed, cracked, glazed or oil-soaked. If the belt is more then five years old, replacement should be recommended to minimize the risk of a belt failure. Adjust belt tension to manufacturer’s specifications using a belt gauge. If the vehicle has a serpentine belt, be sure to check the automatic tensioner to make sure it is functioning properly and that the tensioner’s travel is still within an acceptable range.

The power steering pump should also be checked to make sure it is producing the proper line pressure and that the fluid is operating at a temperature of 170 degrees F or less. Most Saginaw pumps should generate 1,000 to 1,200 pounds of line pressure. If the fluid is running over 170 degrees F, it may indicate a restriction in the return line or the need to install an aftermarket fluid cooler. Fluid that runs too hot can shorten pump life by as much as 50 percent!