Summary and forward and reverse efficiency

efficiency of a screw and nut mechanism is relatively high in the forward direction since the input shaft screw thread inclined plane angle is small. Therefore a very large wedge action takes place in the forward direction. In the reverse direction, taking the input to be at the steering box drop arm end, the nut threads are made to push against the steering shaft screw threads, which in this sense makes the inclined plane angle very large, thus reducing the wedge advantage. Considerable axial force on the nut is necessary to rotate the steering shaft screw in the reverse direction, hence the reverse efficiency of the screw and nut is much lower than the forward efficiency.

9.1.3 Cam and peg steering gearbox (Fig. 9.6) With this type of steering box mechanism the conventional screw is replaced by a cylindrical shaft supported between two angular contact ball bearings (Fig. 9.6). Generated onto its surface between the bearings is a deep spiral groove, usually with a variable pitch. The groove has a tapered side wall profile which narrows towards the bottom.

Positioned half-way along the cam is an integral rocker arm and shaft. Mounted at the free end of the rocker arm is a conical peg which engages the tapered sides of the groove. When the camshaft is rotated by the steering wheel and shaft, one side of the spiral groove will screw the peg axially forward or backward, this depending upon the direction the cam turns. As a result the rocker arm is forced to pivot about its shaft axis and transfers a similar angular motion to the drop arm which is attached to the shaft's outer end.

To increase the mechanical advantage of the cam and peg device when the steering is in the straight ahead position, the spiral pitch is generated with the minimum pitch in the mid-position. The pitch progressively increases towards either end of the cam to give more direct steering response at the expense of increased steering effort as the steering approaches full lock.

Preload adjustment of the ball races supporting the cam is provided by changing the thickness of shim between the end plate and housing. Spring loaded oil seals are situated at both the drop arm end of the rocker shaft and at the input end of the camshaft.

Early low efficiency cam and peg steering boxes had the peg pressed directly into a hole drilled in the rocker arm, but to improve efficiency it is usual

Can And Peg Type Steering Gear Box
Fig. 9.6 Cam and peg steering type gearbox

to support the peg with needle rollers assembled inside an enlarged bore machined through the rocker arm. For heavy duty applications, and where size permits, the peg can be mounted in a parallel roller race with a combined radial and thrust ball race positioned at the opposite end to the peg's tapered profile. An alternative high efficiency heavy duty arrangement for supporting the peg uses opposing taper roller bearings mounted directly onto the rocker arm, which is shaped to form the inner tracks of the bearings.

Cam and peg mechanisms have average forward and reverse efficiencies for pegs that are fixed in the rocker arm of 50% and 30% respectively, but needle mounted pegs raise the forward efficiency to 75% and the reverse to 50%.

To obtain the correct depth of peg to cam groove engagement, a rocker shaft end play adjustment screw is made to contact a ground portion of the rocker shaft upper face.

The rocker shaft rotates in a bronze plain bearing at the drop arm end and directly against the bearing bore at the cam end. If higher efficiency is required, the plain bush rocker shaft bearing can be replaced by needle bearings which can raise the efficiency roughly 3-5%.

9.1.4 Worm and roller type steering gearbox

This steering gear consists of an hourglass-shaped worm (sometimes known as the cam) mounted between opposing taper roller bearings, the outer race of which is located in the end plate flange and in a supporting sleeve at the input end of the worm shaft (Fig. 9.7). Shims are provided between the end plates and housing for adjusting the taper roller bearing preload and for centralizing the worm relative to the rocker shaft.

Engaging with the worm teeth is a roller follower which may have two or three teeth. The roller follower is carried on two sets of needle rollers supported on a short steel pin which is located between the fork arm forged integrally with the rocker shaft.

In some designs the needle rollers are replaced by ball races as these not only support radial loads but also end thrust, thereby substantially reducing frictional losses.

fl Oiker shaft

Fig. 9.7 Worm and roller type steering gearbox fl Oiker shaft

Fig. 9.7 Worm and roller type steering gearbox

The rocker shaft is supported on two plain bushes; one located in the steering box and the other in the top cover plate. End thrust in both directions on the rocker shaft is taken by a shouldered screw located in a machined mortise or 'T' slot at one end of the rocker shaft.

To adjust the depth of mesh of the worm and roller (Fig. 9.7), move the steering wheel to the mid-position (half the complete number of turns of the steering wheel from lock to lock), screw in the end thrust shouldered screw until all free movement is taken up and finally tighten the lock nut (offset distance being reduced).

Centralization of the cam in relation to the rocker shaft roller is obtained when there is an equal amount of backlash between the roller and worm at a point half a turn of the steering wheel at either side of the mid-position. Any adjustment necessary is effected by the transference from one end plate to the other of the same shims as those used for the taper bearing preload (i.e. the thickness of shim removed from one end is added to the existing shims at the other).

The forward and reverse efficiencies of the worm roller gear tend to be slightly lower than the cam and peg type of gear (forward 73% and reverse 48%) but these efficiencies depend upon the design to some extent. Higher efficiencies can be obtained by incorporating a needle or taper roller bearing between the rocker shaft and housing instead of the usual plain bush type of bearing.

9.1.5 Recirculating ball nut and rocker lever steering gearbox (Fig. 9.8)

Improvement in efficiency of the simple screw and nut gear reduction is achieved with this design by replacing the male and female screw thread by semicircular grooves machined spirally onto the input shaft and inside the bore of the half nut and then lodging a ring of steel balls between the internal and external grooves within the nut assembly (Fig. 9.8).

The portion of the shaft with the spiral groove is known as the worm. It has a single start left hand spiral for right hand drive steering and a right hand spiral for left hand drive vehicles.

Worm And Wheel Steering
Fig. 9.8 Recirculating ball nut and rocker lever steering type gearbox

The worm shaft is supported between two sets of ball races assembled at either end normally in an aluminium housing. Steel shims sandwiched between the detachable plate at the input end of the shaft provide adjustment of the bearing preload. Situated on the inside of the end plate is a spring loaded lip seal which contacts the smooth surface portion of the worm shaft.

Assembled to the worm is a half nut with a detachable semicircular transfer tube secured to the nut by a retainer and two bolts. The passage formed by the grooves and transfer tube is fitted with steel balls which are free to circulate when the worm shaft is rotated.

The half nut has an extended tower made up of a conical seat and a spigot pin. When assembled, the conical seat engages with the bevel forks of the rocker lever, whereas a roller on the nut spigot engages a guide slot machined parallel to the worm axis in the top cover plate. When the worm shaft is rotated, the spigot roller engaged in its elongated slot prevents the nut turning. Movement of the nut along the worm will result in a similar axial displacement for the spigot roller within its slot.

End float of the rocker lever shaft is controlled by a spring loaded plunger which presses the rocker

Fig. 9.9 Recirculating ball rack and sector steering gearbox

lever bevel forks against the conical seat of the half nut.

The rocker lever shaft is supported directly in the bore of the housing material at the worm end but a bronze bush is incorporated in the housing at the drop arm end of the shaft to provide adequate support and to minimize wear. An oil seal is fitted just inside the bore entrance of the rocker shaft to retain the lubricant within the steering box housing.

The worm shaft has parallel serrations for the attachment of the steering shaft, whereas the rocker shaft to drop arm joint is attached by a serrated taper shank as this provides a more secure attachment.

Forward and reverse efficiencies for this type of recirculating ball and rocker lever gear is approximately 80% and 60% respectively.

9.1.6 Recirculating ball rack and sector steering gearbox (Fig. 9.9)

To reduce friction the conventional screw and nut threads are replaced by semicircular spiral grooves (Fig. 9.9). These grooves are machined externally around and along the cylindrically shaped shaft which is known as the worm and a similar groove is machined internally through the bore of the nut.

Worm And Roller Steering Gear

Engagement of the worm and nut is obtained by lodging a series of steel balls between the two sets of matching semicircular spiral grooves.

There are two separate ball circuits within the ball nut, and when the steering wheel and worm rotates, the balls roll in the grooves against the nut. This causes the nut to move along the worm. Each ball rotates one complete loop around the worm after which it enters a ball return guide. The guide deflects the balls away from the grooved passages so that they move diagonally across the back of the nut. They are then redirected again into the grooved passages on the other side of the nut.

One outer face of the rectangular nut is machined in the shape of teeth forming a gear rack. Motion from the nut is transferred to the drop arm via a toothed sector shaft which meshes with the rack teeth, so that the linear movement of the nut is converted back to a rotary motion by the sector and shaft.

An advantage of this type of steering gear is that the rack and sector provides the drop arm with a larger angular movement than most other types of mechanisms which may be an essential feature for some vehicle applications. Because of the additional rack and sector second stage gear reduction, the overall forward and reverse efficiencies are slightly lower than other recirculating ball mechanisms. Typical values for forward and reverse efficiencies would be 70% and 45% respectively.

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