CHAPTER VITHE DRIVE
The term used to designate the transmission of power from the engine to the wheels, is called thedrive.
In nearly all cars the engine shaft runs fore and aft, and consequently is at right angles to the axles. This, of course, necessitates some sort of gearing between the engine shaft and axle. This change is made in the bevel gear drive hereafter explained.
As the engine is mounted on the frame of the car, which rests on springs, and the axle is below the springs, it is obvious that the drive must be transmitted between two parts which have a relative up and down movement.
This necessitates several things, structurally, which should be considered.
First. A flexible joint must be interposed in the system, where a shaft is used to transmit the power.
Second. Torsion rods are necessary to prevent the housing or casing of the rear axle fromturning, due to reaction of the driving bevel gear.
Third. A rod, or rods, are required to prevent a fore and aft movement of the rear axle. The rods run from the ends of the rear axle housing to some convenient point on the frame.
Illustrating Power Transmission.—For convenience, these mechanical elements are illustrated on a frame.
Fig. 35. Radius Rods.
Fig. 35. Radius Rods.
Fig. 35 shows a frame which has its rear axle provided with a pair of radius rods A A. These have their rear ends attached, in any suitable manner, to the axle housing, near the springs and the forward ends are brought forward and pivoted to the cross beam B.
Torsion Rod.—These rods thus take care of any undue strain which takes place by the wheel striking obstructions.
C represents the torsion rod which has its rear end firmly secured to the housing D, and its forward end to the cross piece E. This prevents the housing from turning, and also serves to provideagainst any undue thrust of the driving bevel.
Some cars dispense with the torsion rod, by incasing the shaft in a torsion tube. Such a form of construction is shown in Fig. 36.
The torque tubeA, as it is called, is rigidly secured to the housing B, of the rear axle, the forward end being pivoted to a cross piece C of the frame.
Fig. 36. Torque Tube.
Fig. 36. Torque Tube.
The radius rods D D, have their forward ends attached to a sleeve E, located near the forward end of the torque tube A, and the rear ends are secured to the axle housing F at the spring seats.
Some manufacturers avoid the use of these radius rods by such a construction in the springs as will prevent any forward and rearward movement of the axle.
Chain Drive.—The chain drive machines require the radius rods, or some other means to counteract the movement of the axle when it meets an obstruction, particularly where the chaintransmits the power to the differential on the wheel shaft.
Jackshaft.—With the double chain drive no differential is used on the axle, but, instead thereof, it is placed on the jackshaft which carries the small driving sprocket wheels. The chain transmits the power direct to each wheel, and a radius rod is necessary to hold the shaft of the drive sprocket wheel the proper distance from the rear axle.
Fig. 37. Chain Drive.
Fig. 37. Chain Drive.
Such an arrangement is shown in Fig. 37, in which the drive, or jackshaft A is mounted transversely across the vertically-movable frame B, and the torque bar C, therefore, serves as the means for keeping the jackshaft and the axle D the proper distance apart, and it is also arranged to serve as a radius rod to prevent any undue tension on the chain when a wheel strikes an obstruction.
The wheels of such a truck turn freely on the axle stubs of a dead axle.
Objections to Chains.—Few pleasure cars are now equipped in this manner, as the shaft drive is more desirable for several reasons: The use of chains is always objectionable, as the efficiency decreases with wear quicker than the shaft drive, and requires the jackshaft, sprocket chains and sprocket wheels, besides the noise and excessive wear, by stretching of chains, which are always inherent in the use of chains.
It is impossible to prevent dirt, sand and grit from adhering to the chains, unless they are inclosed, a thing which is difficult and expensive. If they are not so protected the lubricant only serves to catch the grit and retain it, so that when it is carried around by the chain, the wheel and chain are both worn out.
Another difficulty in the use of chains is due to the inability to keep them at a proper tension at all times. All chains will stretch in use, consequently the tension will change, and when wear takes place, the distance of the centers of driving and driven sprockets has to be adjusted, calling again for another mechanical complication.
Shaft Drive.—The shaft of the engine, being on the frame, has a vertical movement, and the axle, to which power is to be transmitted, is below.The engine must be mounted so the shaft inclines, or, be placed low enough, so that it will be on a direct line with the rear shaft.
In either case some flexible means must be provided between the engine shaft and axle on account of the relative vertical motion between engine and rear axle. Thestraight line driveis most desirable, in every way, as the full power of the engine is available, and this is usually arranged for by lowering the engine bed sufficiently so that the shaft will point straight to the axle when the car is loaded.
Fig. 38. Shaft Drive.
Fig. 38. Shaft Drive.
Fig. 39. Straight Line Drive.
Fig. 39. Straight Line Drive.
Train of Shafting.—Several lengths of shafting are often interposed between the engine shaft and axle, and some cars have two universal jointsin the shaft line, one mounted forward of the transmission case and the other to the rear of it. Or, more frequently, one in the rear of the transmission and one in front of the rear axle.
It seems, however, to be the most general practice to have a single universal joint directly behind the gear case, and the shaft forward of the case only slightly inclined.
Figs. 38 and 39, show the two types, the former being the straight line drive, and the latter a form of construction where the two universal joints make the drive through a line which minimizes the angles as much as possible between the shafts.
Figs. 38 and 39 are not intended to show all the elements in the train of shafting, such as joints and connections, but is merely designed to illustrate the disposition of the drive shafting relative to the engine and rear axle.