Steering System Designs You Should Consider

CARPHOTO-3047

There really are only two basic categories of steering system today; those that have pitman arms using a steering ‘box’ and those that don’t. Older cars and some current trucks use pitman arms, so for the sake of completeness, I’ve documented some common types. Newer cars and unibody light-duty trucks typically all use some derivative of rack and pinion steering.

Pitman arm components

Pitman arm mechanisms have a steering ‘box’ where the shaft from the steering wheel comes in along with a lever arm comes out – the pitman arm. This pitman arm is connected to the track rod or centre link, which is supported by idler arms. The tie rods connect to the track rod. There are a large number of variations of the actual mechanical linkage from direct-link where the pitman arm is connected directly to the track rod, to compound linkages where it is associated with one end of the steering system or perhaps the track rod via other rods. The example here shows a compound link (left).

Before the front wheels begin to turn, many of the steering box mechanisms that drive the pitman arm have a ‘dead spot’ inside the centre from the steering where you could turn the steering wheel a small amount. This slack can normally be adjusted by using a screw mechanism but it can’t ever be eliminated. The traditional benefit from these systems is that they give bigger mechanical advantage and thus work well on heavier vehicles. With the development of power steering, that has become a moot point and the steering system design is now more to do with mechanical weight, price and design. The following are the 4 basic types of steering box used in pitman arm systems.

Worm and sector

Worm and sector pitman arm steering box

In this sort of steering box, the end of your shaft from your steering wheel includes a worm gear attached to it. It meshes directly having a sector gear (so called because it’s a section of a full gear wheel). When the controls is turned, the shaft turns the worm gear, and the sector gear pivots around its axis as its teeth are moved across the worm gear. The sector gear is mounted on the cross shaft which passes through the steering box and out the bottom where it is splined, and also the the pitman arm is attached to the splines. When the sector gear turns, it turns the cross shaft, which turns the pitman arm, giving the output motion that is fed into the mechanical linkage around the track rod. The following diagram shows the active components that are present inside the sector and worm steering box. The package itself is sealed and filled with grease.

Worm and roller

Worm and roller pitman arm steering box

The worm and roller steering box is similar in design to the sector and worm box. The difference the following is that as opposed to having a sector gear that meshes with all the worm gear, there is a roller instead. The roller is attached to a roller bearing shaft and is held captive on the end of your cross shaft. As the worm gear turns, the roller is forced to move along it but because it is held captive on the cross shaft, it twists the cross shaft. Typically in these designs, the worm gear is actually an hourglass shape so that it is wider at the ends. Without the hourglass shape, the roller might disengage from this at the extents of its travel.

Worm and nut or recirculating ball

Worm and nut pitman arm steering box

This can be by far the most everyday sort of steering box for pitman arm systems. In a recirculating ball steering box, the worm drive has many more turns on it with a finer pitch. A box or nut is clamped on the worm drive that contains lots of ball bearings. These loop around the worm drive then out right into a recirculating channel within the nut where they may be fed back into the worm drive again. Hence recirculating. As the steering wheel is turned, the worm drive turns and forces the ball bearings to press against the channel inside the nut. This forces the nut to go along the worm drive. The nut itself has several gear teeth cast in to the outside of it and these mesh with the teeth on a sector gear which can be attached to the cross shaft the same as in the sector and worm mechanism. This system has much less free play or slack within it than the other designs, hence why it’s used the most. The example below shows a recirculating ball mechanism with the nut shown in cutaway in order to see the ball bearings along with the recirculation channel.

Cam and lever

lever and Cam pitman arm steering box

Cam and lever steering boxes are nearly the same as sector and worm steering boxes. The worm drive is regarded as a cam and has a much shallower pitch and the sector gear is replaced with two studs that sit in the cam channels. The studs slide along the cam channels which forces the cross shaft to rotate, turning the pitman arm, as the worm gear is turned. Among the design features of this style is that it turns the cross shaft 90° to the normal so it exits through the side of the steering box as opposed to the bottom. This may result in a very compact design when necessary.

Steering System designs : Rack and pinion

Rack and pinion steering components Rack and pinion steering cutaway

This is certainly by far the most everyday sort of steering you’ll find in any car today due to it’s relative simplicity and inexpensive. pinion and Rack systems give a much better feel for the driver, there isn’t the slop or slack associated with steering box pitman arm type systems. Unlike those pinion, rack and systems designs have no adjustability within them, so once they wear beyond a certain mechanical tolerance, they need replacing completely,. That’s the down-side. This is rare though.

In a rack and pinion system, the track rod is replaced with the steering rack which is a long, toothed bar with the tie rods attached to each end. On the end of the steering shaft there exists a simple pinion gear that meshes with the rack. Whenever you turn the steering wheel, the pinion gear turns, and moves the rack from left to right. Changing the size of the pinion gear alters the steering ratio. It really is so simple. The diagrams here show a good example rack and pinion system (left) and also a close-up cutaway of the steering rack itself (right).

Variable-ratio rack and pinion steering

This is a simple variation on the above design. All the components are the same, and it all works the same other than the spacing of the teeth on the rack varies depending on how close to the centre of the rack they are. In the middle, the teeth are spaced close together to give slight steering for your first part of the turn – good for not oversteering at speed. As being the teeth get further outside the centre, they increase in spacing slightly so that the wheels turn more for the same turn of the steering wheel towards full lock. Simple.

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