Transmission System 2- نظام نقل القدرة 2

Transmission System 2- نظام نقل القدرة 2

Universal Joints

A universal joint is a mechanical connection between two shafts with interconnecting axes. It provides positive drive whilst allowing angular movement of one or both shafts.
Hooke’s joint:
The Hooke’s type of joint or cross arrangement is widely used, and consists of a four-legged spider or cross fitting into Y-shaped yokes on each shaft, needle rollers normally being used to reduce friction.

Speed variation of a Hooke-type coupling:
These joints do not transmit the drive at uniform speed when deflected. During each revolution the driven shat is subject to two accelerations and two decelerations whose values depend upon the angle of deflection — usually limited to 20°.

When an intermediate shaft, e.g. a propeller shaft, has Hook’s joints at each end, provided the yokes and the angles of the deflection at each are equal, the cyclic variation of the intermediate shaft will be cancelled out by the second joint. The speed fluctuation of the intermediate shaft remains, and this has resulted in the replacement of the Hook’s joints by constant-velocity joints in many applications.
Double, back-to-bake, Hooke’s joints have been used as constant-velocity joints, but are now replaced by more compact devices.

Constant-velocity joints:

One principle for constant velocity (CV) depends upon the engagement between the two shafts place in the plane bisecting the angle between them. In addition to providing constant velocity these joints will operate at approximately twice the angle of divergence (40-45°) of a Hooke’s joint, and are particularly used for the outboard couplings on front-wheel drive.

The Propeller Shaft

The propeller shaft connects the gearbox to a live rear axle or a sprung rear drive unit for the front engine RWD vehicle. It revolves at engine speed in direct drive, and accurate dynamic balance is essential to prevent vibration.

The critical whirling speed where resonant vibration will occur is inversely proportional to the square of the length. For example, if the maximum speed for a 1 m shaft is 5500 rev/mm, which for a 1.25 m shaft of the same type is only 3500 rev/mm.

In order to obviate vibration and noise the shaft length may be reduced by a rearward extension on the gearbox or a forward extension on the rear drive unit. Alternatively a divided propeller shaft is often used. The propeller shaft is normally tubular in section and is made in a one- or two-piece construction. During manufacture, steel shafts are balanced and during this operation small patches, are spot-welded on the ‘light parts’ of the tube to correct the imbalance.

Some form of ‘sliding’ or ‘telescopic’ joint is required in the propeller shaft assembly, so that it can accommodate itself to small variation in effective length of the drive line.

Transmission tunnel height:

 There are many attempts to lower the car height, which in turn called for some lowering of the body floor line. To provide the lowest possible transmission tunnel height, thereby increasing rear seat foot room.   * The propeller shaft either made of steel or composite material. The composite propeller shaft has the advantage of less weight (50% reduction), high internal shock absorption, and corrosion resistance.

Drive shafts

The drive shafts connect to independently sprung front or rear road wheels. Being on the output side of the final drive unit, they operate with that ratio of torque increase and speed reduction compared with a propeller shaft.

In most cases the shafts are of equal length and typically of 20-25 mm diameter nickel- chromium steel. With a transversely mounted engine-transaxle unit, unequal-length shafts may be required. The longer shaft can be partly of larger-diameter tubular construction to equalize the torsional rigidity and vibration characteristic with the shorter solid shaft.

The final drive and differential

The function of final drive gears:

Final drive gears are incorporated in vehicle driving axles for the following reasons:

·        To provide a right-angle drive from either the propeller shaft, or the gearbox layshaft, to the driven wheels.

• To permit an additional and constant gear reduction in the transmission system. These functions can be performed by bevel or worm gears.

The final drive

The duty of the final drive gears is to gear down the speed to suit the road wheels and redirect the line of drive. A crown wheel and pinion bevel gear having a gear ratio of about 4:1 is commonly used on cars but a lower ratio, e.g. 6:1 is necessary to suit the large road wheels used on commercial vehicles;

v = ωw rw = [(2 π Ne/60) / (ig if)] rw,  TE = Te ηt ig if / rw

Types of bevel gear:

The hypoid gear is in common use today due mainly to the fact that the offset pinion allows the propeller shaft to be set below, (for cars) or above, (for commercial vehicles) the crown wheel centre. This gives either a reduction in the/propeller shaft tunnel, which causes a bump in the vehicle floor, or in the case of a commercial vehicle a reduction in the angle through which the universal joint has to operate.

Worm and wheel:

Today this expensive form of drive is rarely used as a final drive on light vehicles, but it is still used on heavy vehicles. Various arrangements, as shown in the figure can be used to give a very quiet and long-lasting gear, but efficiency is not as good as with the bevel (94 per cent against 98 per cent).

The back axle ratio (final drive ratio) (if):

With the actual gears he number of teeth on the crown wheel (the large cone) divided by the number of teeth on the bevel pinion (the small cone) gives the ‘axle ratio’. Also the gear ratio of a worm and wheel is given by dividing the number of teeth on wheel by the number of starts on worm.

It should be noted that a ‘high ratio’ axle refers in fact to one with a low numerical ratio. For example, a final drive with an axle ratio of 4.10:1 is a ‘high ratio’ axle as compared to one providing 4.56:1 ratio. In this connection, it will be observed that the axle ration appears to be somewhat ‘fussy’ figure, such as those just quoted. This is because the axle designer prefers to use an odd number of teeth on the pinion so that each tooth on the crown wheel engages every tooth on the pinion in regular succession.

Differentials with a low (numerically high) gear ratio allow for fast acceleration and good pulling power. Differential with high gear ratios allow the engine to run slower at any given speed, resulting in better fuel conservation.

Special transmission units:

a) Two speed axle; this is a convenient method of providing a large number of gear ratios and retaining a light gearbox. The torque applied to the final drive unit is also moderate as the extra reduction and the increased torque are mad after the crown wheel and pinion

Tw (1st speed) = Te ig ηt i1,       Tw (2nd speed) = Te ig ηt i2

where: i1, i2 are the 1st and 2nd axle speeds respectively, and ηt is the transmission efficiency.

b) Double reduction axle; two reductions within the same unit

Tw = Te ig ηt if  = Te ig ηt ia ib

where: ia, ib are the 1st and 2nd reductions respectively.

c) Hub reduction; a reduction made at the wheel hub reduces the stresses which would be applied to the final drive unit having a large reduction ration. Fore example if a 2:1 hub reduction is made this halves the torque to be transmitted to the axle shafts, differential, crown wheel and pinion, propeller shaft and gearbox.

Tw = Te ig ηt if ih

where: if, ih are the back axle ratio and hub reduction respectively.

The differential

The differential provides an equal torque to each half-shaft or drive shaft although they may be rotating at different speeds. It therefore allows the outer road wheel to revolve faster than the inner when cornering, whilst maintaining a positive drive to both wheels.

Since no grater torque can be transmitted to one road wheel than the other owing to the balance action of the differential, if one road wheel is on a slippery surface where it can revolve idly, no tractive force can be applied to the other wheel.

The limitation of the differential -loss of drive if one wheel spines- can be eliminated by the use of a differential lock, e.g. dog clutching one of the sun pinions to the differential cage. Alternatively, a limited-slip differential (LSD) arrangement with solid or viscous friction between the sun wheel and the differential cage will apply some torque to the wheel having grip.

* The torque is divided between the two wheels. The speed of inner and outer wheels can be obtained from the following:

Let Nc = rev/min of crown wheel or differential unit, Ni = rev/min of inner wheel, and No = rev/min .of outer wheel. Then

Nc = (Ni + No) /2

and              No = (Nc x2) – Ni  or      Ni = (Nc x 2) - No

Rear axle construction

In cases where the rear suspension is non-independent, the type of axle used is either a dead axle or a live axle. The former only has to support the weight of the vehicle, where the latter has to fulfill this task and, in addition, contain a gear and shaft mechanism to drive the road wheels.

Axle shafts:

The axle shaft (half shaft) transmits the drive from the differential sun wheel to the rear hub.

The various types may be compared by considering the stresses the shaft has to resist. The half shaft subjects to the following stresses:

 1- Torsional stress due to driving and braking torque. 2- Shear stress due to the weight of the vehicle. 3- Bending stress due to the vehicle. 4- Tensile and compressive stress due to cornering forces.

The types of axle shaft:

In addition to their other features of general construction, driving rear axles are classified into three groups depending on the type of bearing mounting used to support the hubs. The three arrangements classified as follows:

 • Semi-floating • Three-quarter floating • Fully floating

Semi-floating rear hub:

A single bearing at the hub end is fitted between the shaft and the casing, so the shaft will have to resist all the stresses (shear, bending, torsional). In this arrangement, if the axle shaft breaks, the driving wheel comes away from or out of the axle housing.

Three-quarter-floating rear hub:

In this construction the single bearing is located between the hub and the outside of the axle casing. This relieves the shaft of shear load and bending loads due to the vehicle’s weight, but it is still subject to bending loads due to cornering side thrust, and torque. If the shaft fails, the wheel will still be supported but side loads may cause it to rock on the bearing.

Fully floating rear hub:

In this construction the axle is supported by double taper-roller bearings on the outside of the axle casing. In the frilly floating construction the axle shaft transmits driving torque alone. The axle removal or failure does not affect the road wheel, and the disabled vehicle can be towed to a service area to for replacement of the axle shaft. This system is generally used with heavy vehicles.

Tires

Types of tires:

The basic variations in motor vehicle tire construction may be considered under the following headings:

• Tube and tubeless

A separate rubber inner tube acting as a flexible bag to retain air introduced under pressure early became an established feature of pneumatic tire Since mid nineteen fifties it has become established practice for private cars, and to a lesser extent for commercial vehicle, to dispense with the separate inner tube in favor of an inherently air-tight or ‘tubeless tire’). The tubeless tire has the following advantages: (increased safety, cooler running, and Improved balance).

• Cross ply and radial play

The radial ply tire is relatively free from internal friction because of the non-criss-crossing of its carcass plies. This in turn means cooler running and it permits reduced inflation pressure for greater cushioning ability.

Tire aspect ratio:

 It is the ratio of the tire inflated section height to section width for a specified rim width. A modern radial ply tire with a ‘low’ aspect ratio is both wider and shallower than an old fashioned cross ply tire with a ‘high’ aspect ratio. The ratio is usually expressed as a percentage.