Last week I mentioned the steering box as one of the key transition points between the driver’s arm efforts and the resultant directional change of the motor vehicle he is driving.
Its importance comes from the fact that the steering box (also sometimes referred to as the rack) is the device that differentiates the rotational motion of the steering wheel into the linear motion of the linkages (the track rod, actually) at the wheels.
Besides this, the rack/box also gears down the driver input to multiply the torque, or the twisting force of his arms if you will, and makes his work less physically demanding by providing a high ratio reduction.
However, a rack is not just a rack. There are types, and these types have differences and characteristics unique to each one, physical or otherwise.
It is with these steering box types that we open our Wednesday classroom today.
Rack and pinion steering: This is the simplest setup to visualise and build, and is therefore the most common steering type used in the world today.
As the name suggests, there is a rack and a pinion, though in this case, the word rack does not refer to the whole box as described above, but to a flat, slatted surface, rather like a dish rack.
The pinion is, well, a pinion, which is another name for a lone, freestanding cog or toothed wheel.
The cog is the terminus of the steering column, so when the driver turns the steering wheel, the cog rotates too, sans any gear reduction.
The pinion is meshed to the rack, so the pinion’s teeth in circular motion induce side-toside motion in the rack, just the same way as motion is induced when one gear meshed to another starts rotating.
It is at this point that the gear reduction in the system occurs. As with anything, there are pros and cons to doing something in one way rather than the other.
Rack and pinion helms have good steering response, they offer greater resistance to play and the “dead” on-centre feel (the sloppiness around the center before the wheel is turned), they also offer improved steering “feel”, or better feedback, and create lesser backlash (those who have had their thumbs knocked off piloting unassisted-steering 4WD trucks over rough grounds know what this is).
Recirculating ball steering: This type is a bit harder to fathom, even with diagrams. It contains something called a worm gear, which is not entirely dissimilar to the threaded section of a regular bolt or screw.
This worm is found inside a block which too has a threaded hole in it (the worm goes into this hole, just like they do with apples).
The block also has teeth cut into its exterior, and these teeth engage a sector gear that is connected to the Pitman arm.
When you cut a pie or a pizza into pieces, these pieces are mathematically referred to as sectors. Therefore, it follows that a sector gear
is a “piece of gear”.
The Pitman arm is what causes the road wheels to turn when the steering wheel is turned. The steering wheel shaft is connected to the worm gear (think of the worm as the very end of the shaft, the same way as the pinion is in the R&P steering above), and this shaft rotates the worm gear within the block.
As with all threaded joints, rotation of one piece causes movement of another (or of the same piece), and in this setup the worm is set in such a way that it is stationary, so its housing, the block, is the one which moves.
The motion of this block is transferred to the Pitman arm via the sector gear, and the Pitman arm in turn causes the road wheels to turn.
Variable Ratio steering: This is a steering system with non-constant tooth pitch (number of teeth per inch) on the rack from the centre outwards.
It is common with rack and pinion steering systems because it is easy to modify the tooth pitch on the rack. The steering ratio is quite easy to calculate.
It must have come to your notice that most cars have between three and four turns lock-to-lock (from one end, through dead centre to the other).
One complete revolution of the steering wheel is 360 degrees. Check for the corresponding angle change in the front tyres. If it has turned 20 degrees, then the steering ratio is 18:1 (divide 360 by 20).
If the tyres have a maximum deflection of 25 degrees either way (25 to the left and 25 to the right), with the 18:1 rack, then from lock to lock the tyres go through 50 degrees, but with the ratio, the steering wheel turns 900 degrees, which gives 2.5 turns lock to lock.