An interesting problem in connection with commercial car designing which merits careful consideration is that of mounting and arranging the power plant so as to protect it from stresses caused by frame weaving, due to road irregularities. Vibration is another factor of considerable magnitude that must be considered, while provision must also be made for torque reaction.
Power-plant mounting is being freely discussed and there seems to be a general tendency toward some form of flexible support, so that sufficient freedom is given the engine, while others resort to a spring mounting, which combined with a flexible support, protects the power plant from vibration and frame weaving.
Opinions differ greatly as to the correct mounting, some maintaining that the usual method of bolting down the two rear engine arms rigidly to the side rails of the frame does not give the engine sufficient freedom, even if a flexible support is provided at the forward end. Others take the opposite view, claiming that the front flexible support is sufficient. There are also some engineers who claim good results can be obtained by a rigid central support at the front end, as this practically gives a three-point support, and permits frame weaving to be taken up by the cross member which supports the forward end of the engine.
The material give of the cross member should absorb severe stresses and also hold the engine more rigidly against torque reaction.
If the engine is mounted with a pivoted support at the forward end, the torque reaction caused by an explosion in the front cylinder, must be transmitted through the crankcase to the rear engine arms before it reaches the frame. However, if the forward support is of the rigid type, the stress goes to the frame direct.
In addition to the flexible front support, some makers also provide swivel supports for the rear arms, so that all the torque reaction must go to the rear arms, but no frame distortion can by any possibility put a stress upon the crankcase. Larger and more massive engine arms are also being used, thus increasing the efficiency of the present mountings.
Coil springs of considerable strength are also used under the front or rear supports, and these absorb some of the stress created by frame weaving, while they can also be arranged to absorb some vibration.
An important point with a rigid mounting is the method of securing the rear arms to the side rails of the frame. In this case, the engine must be held securely, and the frame must not be appreciably weakened, while the arrangement must be such that the supporting arms can be quickly freed when it is desired to remove the engine from the chassis.
In the unit power plant the transmission is supported from the flywheel housing, but in the amidship position, it is usually mounted on a three-point support, so that it has a certain degree of flexibility to resist frame weaving. In some cases where a flexible subframe is used for the motor, this is also arranged to support the transmission. For midship mounting, cross members of the frame are usually used, so that the forward support forms the flexible member, while the rear carries the two rigid supports.
One of the most prominent types of flexible supports is shown in Fig. 194, which may be adapted to either the engine or a unit power plant. This particular illustration represents the Globe 1-ton truck, equipped with a Continental engine. In this construction two cast arms integral with the flywheel housing, form the two rigid points of support. These are set on hangers, riveted to the side rails of the frame, while holts pass vertically through both, to hold the power plant in position.
The third point of support is at the front end of the engine, and consists of a bracket fitting over a finished surface, on the hub extension of the gear cover plate. A cross member passes under this, and has the bracket fastened to it by two holts.
This engine is also used on the Denby trucks, and is mounted in a similar manner, but in order to provide more flexible rear supports, one bolt on each side is fitted snugly and provided with a coil spring, the others being a loose fit.
Fig. 194. A Prominent Type of Flexible Support, which may be Adapted to Either the Engine or a Unit Tower Plant.
Fig. 195. A Three-Point Main Frame Mounting Employed on the Riker Trucks.
Another type of three-point mainframe mounting is shown in Fig. 105, being employed on the Riker commercial cars. In this construction, a heavy drop forged member is attached to the crankcase at the war by studs, which pass clear through the case. These studs are so close together that considerable freedom is obtained by this supporting member, through the elastic extension of the studs, and the elasticity of the forged member. The forward end is also supported by a forged member; however, this is pivot ally arranged in a bracket bolted to the crankcase. Metal filling blocks are fitted into the side rails of the frame, and three bolts in each end of these supports secure the engine to the frame. The top flange of the supports overhangs the filling blocks, and so relieves the holts from the weight of the engine.
The Pierce 5-ton truck engine is also mounted in a similar manner, while the Packard truck engines have a pivot mounting at the front end, and the rear end is supported by a large member which is boiled to the flywheel housing.
An interesting and simple method of support is used on the Union trucks (Fig. 196), which is covered by patent. The forward support is of swivel type, consisting of a bracket fitting over a hub extension of the timing gear cover. This bracket has two lugs which rest on the upper flange of the cross member, so that the weight is taken off the bolts that hold the bracket in position.