AT THE HEART OF THE ROAD TRANSPORT INDUSTRY.

Call our Sales Team on 0208 912 2120

Is this the PERFECT ENGIN DUNTING SYSTEM.?

28th January 1949
Page 12
Page 13
Page 14
Page 12, 28th January 1949 — Is this the PERFECT ENGIN DUNTING SYSTEM.?
Close
Noticed an error?
If you've noticed an error in this article please click here to report it so we can fix it.

Which of the following most accurately describes the problem?

By A. J. HIRST, B.Sc., A.M.I.Mech.E.

HE development of vehicle-engine mounting on sound engineering lines starts from an appreciation

of the importance. of the principal' axis. It is generally known that .this-axis,..about Which' the engine

should be , free to oscillate; runs from fairljt high up on the front of the cylinder block to the coupling behind the clutch. housing or: gearbox, but .even among . vehicle • designers its significance is not always . appreciated. Actually, there is nothing in the design of an engine mounting that need present difficulty to the average engineer.

Finding the ." Principal Axis"

If one can imagine the power unit set up between . centres on a lathe, with its centre line running through

, the centre of gravity, the conditions on the chassis are easy to follow (Fig. 1.). The power unit, as carried, is in balance statically, and if rotated slowly will have no tendency to remain in any one position. On the other hand, it :does not follow that the. power unit can be rotated rapidly in balance. For instance, the.greater • part of the weight at the front end might be to one side , of the centre line, and the weight of the rear end to the other side The effect is that of unbalanced weights. in opposite directions at the headstock and ,tailstock ends. . If the assembly be rotated rapidly,: these weights will pull.on the two centres. in opposite directions and cause vibration This merely indicates that the set-up is not " dynamically balanced "; a crankshaft, for instance, may be in balance on knife. edges (statically) and still set up vibration when in motion With a crankshaft, the.cure. is, of eours.2, to add, ortake away, weight, in diametrically _opposite positions .at the. two ends of the shaft, but in balancing a power' unitfor mounting, there is a more corn. en iept method • The direction of the

• axis may be adjusted . so that the : effect of opposite unbalance at the two ends is eliminated. .

Such an arrangement is shown in . Fig. 2. Here the power unit remains in balance :if it be rotated. about this inclined centre line, Which is " principal axis.". it is also in balance if it be oscillated about the same axis, and in neither case is there any loading of the centres additional to that due to the dead weight carried.: Actually ; there , are three principal axes, all • pernendictilar,A6 one another, but the one running approximately 7for,e and aft is that . about which the engine is mounted. ' The first commercial application of this principleas worked out by the Chrysler. Corporation in America. The 'patent specification shows mountings Of rubber bonded to metal, positioned on the principal axis of a large four-cylindered petrol engine. These:incitintingS offer practically no resistance to torque reaction, so a light leafspring was attached to the underside .of the clutch housing, with its free , end mOnIcled into a rubber block attached to the chassis, frame. A mounting on exactly the same principle, but attached to a six-cylindered oil engine, is shown in Fig. 3.

Absorbing Engine Vibration The practical value of Mounting on the principal axis is readily.. appreciated. Oscillation about this Otis produces no reactions at the mounting Point, except those due to the slight torsional stiffness of the rubber units : Steady torque reaction is earned by the flexible leafspring, whilst 'vibration" is absorbed by the inertia

of the oscillating engine.' •

Designs similar to Fig. 3 are .still used with success irs Ainerica indeed there are many Chrysler and Plymouth cars, built about 19.30, still running with the original engine :mOuntings, a striking tribute to the durability of the rubber and its bond to metal if a mounting on the same lines were designed to day then some detail alterations would be made in the rubber form. More simple shapes of metal parts, which would be easier to Mould and test for sound bond and less liable to stress concentration, 1;vould he preferred. Nevertheless, the rear mounting shown in Fig. 3 has

given good service, and the metal parts are shaped to give the fore and aft stiffness necessary to eliminate clutch shudder.

Floating power, in its original form of construction, has not been used in this country, but all successful flexible mountings are the same in principle, and the alterations were made more from the point of easy installation, especially with existing engines, than for• efficient working.

A near equivalent to the original floating-power design, which is still giving satisfactory service in this country, is found in the installation of the A.F.C. 9.6-litre engine with a separate gearbox. In this case the rear mounting takes the form of a complete ring of rubber clamped round an extension of the clutch housing. It is not bonded, but relies upon friction to keep the engine in its upright position. Being of large diameter, its stiffness is sufficient to resist the nortnal torque of the engine, but small rubber buffers are added to limit movement under shocks arising from starting and stopping. The front mounting is another rubber ring, but of much smaller size and acting, in practice, as only a hinge.

ln all floating-power designs the axis passes closely through the driving coupling at the back of the engine. It would appear that the inclined axis was originally worked out to keep the drive steady, rather than to comply with any principle of mechanics. Subsequent research has left no doubt, however, that the principal

axis of inertia is the fundamental feature, whilst it is the fortunate coincidence that this axis passes near to the driving joint which makes floating power such a success. If the flywheel were mounted on the front end of the engine, the problem would be much more difficult.

Most British mass-produced private cars have a form of engine mounting which aims at securing a floatingpower effect without the extra front cross-member. The single front mounting point has been replaced by two, one on each side of the engine, and set in V form. This, in effect, amounts to increasing the radius of the semi-circular type of mounting and cutting away the greater part of it.

The methods of obtaining the effect of single-point mounting from a distance are important enough to merit description in detail. Fig. 4 shows a number of alternative layouts, any one of which gives the effect of a single attachment at the centre of the complete bush. The fundamental property of them all is that a twist applied to the engine support rotates the mounting about the selected centre, and that a vertical or horizontal force gives displacement without any rotation. The geometrical centre of the sector lies above the axis, whilst the intersection of the centre lines of the V mountings lies still farther above. The " correction " of the intersection point-is often omitted, but is necessary to achieve the best results. It compensates for the compressibility of the rubber, and becomes small if the stiffness of the mounting for compression loading of one plate towards the other is much higher than the stiffness for displacement of one plate parallel to the other.

A Passenger Chassis Engine Mounting

The arc-shaped mounting, consisting of rubber bonded between two curved plates, is a usual choice for the single front or rear mounting point. Fig. 5 shows a type that has been used for many years in a passenger chassis. The cut-away in the centre is merely for convenient fixing, but a point that immediately suggests itself is that the radius might be increased and the cut-away lengthened, The sector is then nearly equivalent to a pair of mountings in V-form, bonded with connecting plates into a unit. From this it is an obvious and logical development to make the two mounting units separate and to use flat plates instead of curved ones. The difference between the short sector of large radius and the flat plate is so small as to be of little moment.

By a suitable choice of rubber proportions the Vmounting can be made as flexible as may be desired. The larger the distance from the axis, the more flexible the mounting point must be, so that eventually there conies a point where vertical and horizontal flexibility is apt to be too great. The insulation of vibration may remain good, but the engine may be liable unduly to bounce about on its mountings, especially on' rough

. Some . private-car engine, mountings are deliberately made. too stiff torsionally_ in order to over

. . come bouncing and, therefore, do not insulate properly:

at low engine speeds. When the engine . idles, the impression is sometimes created that the unit is rocking about•one or other of the mountings forming the V.-, '

• The solution is-to decrease the 'Compressibility of

the mounting units by the use of an -intermediate plate in the manner shown in Pig, 4.. This intermediate plate

_

restrains thebulging of the.surfaces•df the rubber and reduces -its compressibility by half. Flexibility for .rotation about the aNis is unirrioaired.This refinement . in design is needed.,only when the mountings must be . s,et at a wide radius, ' • but it is a . great ,iniprovement for an oil-engine installation. V-mouritings may be used front and rear, but most often for the front only, with a single point at the back of the gearbox. The gearbox mounting may. in fact, be in almost arty form, either of rubber bonded to metal or a plain rubber moulding._ It is little more than a hinge, but it .should resist fore and aft loads due to de-clutching, etc. Very often a single-bonded rubber pad is used,

• together with a link locating the power unit against longitudinal movement. .

Fig. 6 shows a V-mounting at the front, together with a good form of rear-engine mciunting that provides all the necessary foi c-and-aft stiffness without a separate link. In other cases a pair of mountings in V-form is placed close together on each side of the gearbox and a link is added parallel to the axis of oscillation. The rubber-to-metal bond bushes may well be used at the ends of the-link.

: Referring again to Fig. 3, the other method of retain

ing the one-point-mounting effect, without special cross-member, is the link construction. The lower ends of the links are carried by brackets from the crankcase and the upper ends on the usual eross:member. It is easy to see that this arrangement leaves the engine free to oscillate aboutthe axis, restrained only -by the torsional stiffness of the link joints and, more important, by the pendulum effect of the links themselves. A front mounting on these lines has been used for many years ,by Dennis Bros.-, Ltd., whilst the Leyland company has adopted a front and rear suspension of this type in :its passenger vehicles. ' Bushes of rubber, bonded to metal, may be used as the_hinges.

The Leyland rear links are hung from the normal arched cross-member, passing over the flywheel housing, and rubber bushings are used at all joints. As in the ease of the V-mounting, there is a correction of the intersection position to allow for the compressibility of

Tags

People: A. J. HIRST
Locations: Plymouth, Leyland

comments powered by Disqus