THE

BURMAN-DOUGLAS

STEERING GEAR

Technical Information & Service instructions.

During the past few years there has been a marked tendency towards emphasizing the importance of the Steering Gear in the design of a Car. The great increase in touring speeds, the use of low pressure tyres, and the influence of the lady driver, have called for a degree of sensitivity and driving comfort which cannot be attained by the type of Steering Gear which has hitherto been considered satisfactory.

This had led to the marketing of several proprietary Steering Gears of varying merit, and we propose, in the course of this short booklet, to show the superiority of the Burman-Douglas from a point of view of theoretical and mechanical construction.

The well-known and basically correct principle of the screw and nut is used, but with many important differences which are set out in detail as follows:

GENERAL DESIGN.

Before dealing with the more technical aspect of the Burman-Douglas Gear, it will be as well to consider the general construction of the Gear. Elsewhere in this booklet will be found a cut-away drawing in which all the parts are shown, and a facsimile of a standard general arrangement, which will be more suitable than the cut-away view for reference purposes when servicing. The two drawings together show all the points referred to in this booklet.

It will be seen that the basis of the Gear is a multi-start screw cut on the Steering Column. This screw is of the well-known acme form, with clearance on top and bottom of thread, so that bearing takes place on the flanks only. The nut is free to move longitudinally in the main Steering Box housing, while in the side of the nut is a hardened steel bush in which operates a ball headed extension of the Rocker Shaft. The latter is carried in the main body casting, passing beneath the nut at right angles to it, and connected at the other end to the Drop Arm.

It will be seen, therefore, that when the Steering Column is rotated the nut moves along its axis and is, in turn, controlled by the Rocker Shaft Bail Peg, to which it imparts an arcuate motion, and as the Drop Arm is fitted on the opposite end of the Rocker Shaft, this will follow the same motion, which it imparts to the drag link. All end thrusts are taken by a deep grooved journal race on the top end of the Steering Column, which being some distance from the worm and nut, allows a partial floating action of these parts.

Sliding friction, which must be present in any Gear if proper damping action is to be obtained1 takes place between the flanks of the screw and nut, the area here being ample to ensure low unit pressure and long wear, the projected area being several square inches. It will be noticed, therefore, that one of the main features of the design is the ample bearing area, as opposed to other Gears which have, at best, merely line or spot contact, with consequent breaking down of the oil film and quick wear. At no time can loads be imposed on the Burman-Douglas Steering Gear which will break down the oil film, and, therefore, apart from its long life, it never gives rise to that feeling of temporary seizure which is sometimes experienced on other types of Steering.

BALL AND BUSH.

It should be noted that the function of the ball headed pin that enters the bush in the side of the nut, is to withstand load only. The relative movement which takes place between ball and bush, amounts to only one-fifth of the revolution of the ball during a full lock-to-lock travel of the Steering Gear, and it is so minute that for practical purposes, the bail can be considered as being stationary in its bush, and no measurable wear occurs on the line contact taking the load on this pin, even after very great mileages.

It should also be noted that although the actual relative movement between the ball headed pin and its bush is in no way sufficient to introduce any question of friction, owing to the very slow “in and out” and rocking movements, the load carrying contact area does not occupy the same position at different parts of the gear travel, thus avoiding concentration of load at the same point.

It is in no way comparable with other types of Steering, in which this particular component has to withstand considerable loads, together with sliding friction at high speeds.

ACTION OF THE GEAR.

As previously explained, rotation of the Steering Wheel leads to an arcuate motion of the Rocker Shaft, but it should be noticed that a slightly accelerated movement occurs on the right hand lock, and a slightly decelerated movement on the left hand lock. This tends to equalize the Steering Wheel movement on both locks where the Ackerman layout is used on the front axle. The force applied to the ball headed pin in the rocker arm is also a direct push or pull, and not a moving inclined plane, as in other Gears.

The characteristics of the Gear, as far as self-centering and other points are concerned, depend on the particular layout of different Cars, and to this end, the Burman-Douglas Steering is designed specifically to suit each particular Car to which it is fitted. It is possible, by varying the particular component, to obtain any combination of ratio, mechanical advantage, and efficiency or sell-centering to suit each particular Car.

The first point to be decided is the helix angle, which is fixed with reference to the degree of self-centering required, without permitting excessive road reaction. The ratio required affects the lead of the screw and the mechanical advantage is fixed by the length of the Rocker Arm and the length of the Drop Arm. These two points can be varied to give the mechanical advantage required, and the diameter of the Column is varied so as to give the required helix angle with the ratio which has previously been fixed.

It is thus possible to provide a Steering Gear which wears well and is inherently smooth and light in operation under all conditions.

CONSTRUCTION OF THE GEAR.

There are no bearings in the Steering Box, as the cylindrical nut itself slides with a slight double rotating movement in the barrel of the Steering Box. This sliding fit, which is more than adequate, takes the tilting stresses due to the side engagement of the ball headed pin and relieves the screw from all stresses except end-thrust, which, as previously explained, is taken at the top end of the Column; thus the screw is a true floating assembly in the nut. The ball headed pin is mounted as close to the screw and the nut as possible, and thus any tilting stresses are small. It should be noted that the Inner Column screw and the nut remain co-axial throughout the travel of the Gear, incidentally rendering the fitting of central controls extremely simple, and this, together with the extreme simplicity of the ball and socket connection from nut to rocker arm, gives considerable advantages as regards efficiency and cost when compared with other types of Gears.

SERVICE AND MAINTENANCE.

While Burman-Dougias Steering Gears vary in details on the various Cars to which they are fitted, the main principle in each case is the same and is shown in section in the illustration.

From the service angle, the complete Steering Gear may be divided into three main assemblies, the first being the Outer Column, which is rivetted at one end to the Steering Box itself, which carries the bushes for the Rocker Shaft, and the other end having the outer member of the Bail Race also rivetted to it. The second member is the Inner Column, carrying at the top end the fixed and adjustable inner races and at the bottom end the screw on which is assembled the main nut; the third component being the Rocker Shaft to which is assembled the Ball Peg, and after final assembly into the Steering Column itself, the Drop Arm is secured to it by means of splines and a pinch bolt.

Adjustments.

There is only one adjustment in the Burman-Douglas Steering Gear, which is to the Ball Race at the top of the Column. This is easily effected and in most cases it is possible to reach this point without removal of the Steering Wheel, but in some cases it is necessary to raise the Steering Wheel slightly along its. axis, so as to expose the adjustable and fixed Ball Race nuts which are then easily adjusted by slacking back the locking nut, tightening up the adjustable race which has a hexagon head, until lightness is felt, then slacking this back approximately one-eighth of a turn, then tightening up the locking nut.

It is easy to detect when this race needs adjustment firstly by reason of the back lash which becomes apparent, and secondly. by the up and down movement of the Steering Wheel, which is attached to the Steering Column. It sometimes happens, however, that after a considerable mileage, the back lash may not be sufficient to point to the necessity for adjustment of this race and no perceptible up and down movement can be detected. However, if the Steering Wheel is moved backwards and forwards with the Car at rest, a slight knocking can be detected at the top of the Column and this shows that adjustment of the race is necessary. As a normal rule, adjustment of this race should only be required approximately every 20,000 miles.

Dismantling.

The correct procedure for dismantling a Burman-Douglas Steering Gear is as follows:—

When the Steering Gear has been removed from the Car, the centre controls should be taken off by removing such levers, etc., as may he present at the bottom end of the Steering Column, and these, together with the Steering Wheel, removed as a unit. Next, the lock nut for the adjustable Ball Race at the top of the Column should be removed, together with the adjustable Ball Race immediately beneath it. The balls can then be removed from the outer race and the whole Column by screwing either right or left hand, can be removed from the Steering Assembly. It should be pointed out that the hand of the thread varies with the different types of Steering Gear to suit the particular axle layout on the Car, but it can easily be found by turning the Column whether this has a right or left-hand screw, and act accordingly in removing the Column from the Steering Assembly. Next remove the screws holding the End Plate in position from the end of the Box and the screws holding down the Cover Plate. The Drop Arm can then be removed by unloosing the pinch bolt and gently knocking this out, then removing the Drop Arm from the splines on the Rocker Shaft. It will then be found that the Rocker Shaft complete with Ball Peg can be removed through the Cover Plate and the Main Nut will then be easily withdrawn from the Steering housing.

When reassembling to the chassis, it is important to note that the Drop Arm. is fitted in the correct position. This can easily be found by disconnecting the Drop Arm from the Steering Assembly proper and setting the front wheels of the Car in approximately the straight ahead position. The Steering Wheel should then be turned until it can be turned no further in one direction and then turned carefully in the opposite direction, counting the number of complete turns, until it comes to a stop at the other end. Then commencing from one of these stops, take the Wheel back half the complete number of turns available, this bringing the Steering into the central position. Next, with the Gear in the central position, the front wheels should be set pointing slightly to the left of the straight ahead position and the Drop Arm fixed on its serrations on the Rocker Shaft.

It is important when re-assembling the Steering assembly to the chassis, that the Steering should be correctly aligned and the dash bracket should not be tightened until after the Steering Box is firmly secured to the chassis. The dash bracket must then be adjusted so that the Column is not strained in any direction, either sideways or up or down, otherwise, stiffness in Steering will result, together with undue wear of the moving parts.

Replacement of Parts.

Attention has been directed above to the adjustment of the top Ball Race when play is detected in the Steering Gear. If play is still present after adjustment of the Ball Race, it is in one of the points in the Steering Box itself, This can be either between the flanks of the threads on the worm and nut, or between the Rocker Shaft and its bushes. It is unlikely to be in any other place. In the case of wear taking place between the worm and the nut, we recommend that the complete unit of the two parts should be replaced, the reason for this being that in the course of manufacture, the nut is lapped on to the mating Column, so as to obtain a very high degree of finish without back lash and when wear has taken place, it usually takes places approximately equally on both components, and therefore, it is better to replace the two components as one unit. In certain cases, however, we are prepared to supply replacement nuts, but in that case, it should be remembered that these, when supplied, will be a tight fit on the Column and must be carefully lapped in with the use of a suitable compound, the greatest care being necessary to clean all the surfaces afterwards from any form of abrasive material. If wear has taken place in the Rocker Shaft bushes, these may be removed by tapping. They are merely a press fit in the Steering Gear housing, with the exception of the one furthest from the Drop Arm which, in some cases, is pegged to the housing to prevent rotation. In these cases, the Rocker Shaft should be removed, the peg driven straight through from the outside of the Steering Box into the inside of the bush, and the bush can then be removed by gently tapping.

In certain of our larger Gears, the Rocker Shaft bush furthest from the Drop Arm is replaced by a journal bearing. This is only a press fit in the housing and offers no difficulty in removal. Adjoining the Rocker Shaft bush nearest the Drop Arm is the oil seal which takes one of three types. In one type it is merely a felt washer which is partially recessed in the Steering Gear housing and is held in position by the Drop Arm itself. In the second type, a felt or composition cork washer is pressed into a recess in the Steering Gear housing, which causes it to constrict on to the Rocker Shaft, thus forming an effective oil seal. The cork or felt is held in position by a plain mild steel washer placed over the outer surface and lightly held in position by peening the housing metal over it. If the seal requires replacement, it is a simple matter to knock out the mild steel washer, fit the replacement seal and re-fit the washer. In the third type, a self- contained oil seal is fitted as a complete unit. This is held in position by lightly peening over and can be removed in the same way as described for the type mentioned above.

It should be noted, in connection with replacements, that the ball peg bush is fitted in the nut by a special process and cannot be supplied as a separate component. Also, the ball peg is driven into the Rocker Shaft and rivetted over, and this also cannot be supplied separately.

Lubrication.

The Steering Box should be completely filled through the oil filler plug provided, with a medium bodied gear oil such as Shell Spirax. The only other lubrication point is the Ball Race at the top of the Column, which is packed with grease when the Steering is assembled. This requires no replenishment, but if at any time the Steering is dismantled and this grease cleaned out, when reassembling, it should be refilled with a heavy type of ball bearing grease.

We shall be glad to give any further information on points which may not be dealt with above, or any technical information regarding the performance, design or fitment of the Steering Gear, on request.