Reciprocating internal combustion engines produce an output torque with torsional vibrations. These torsional vibrations cause undesirable impact loads, vibration and noise in vehicles and vehicle drive trains. Vehicles with manual transmissions employ torsional vibration dampers in the clutches which transmit torque from the engine crankshaft to the transmission input shaft. Vehicles with torque converters have relied upon the torque converter to provide hydraulic torsional vibration dampening. However, torque converters frequently include lock-up clutches to reduce power loss during certain operating conditions such as high engine speed operation. When the lock-up clutch is engaged there is minimal hydraulic torsional vibration dampening by the torque converter. Lock-up clutches used in combination with torque converters, therefore, include torsional vibration dampers.
Torsional vibration dampers employing compression springs, to absorb torsional vibrations, frequently have several spring assemblies that are positioned in a circle and are loaded together, parallel to each other. Each of the spring assemblies are loaded in compression between two surfaces that move toward and away from each other in an arc about a fixed point. To prevent failures of coil springs due to buckling, it is desirable to limit the change in the angular position of the surfaces, in contact with the ends of each spring assembly, relative to each other. The angular positions of the surfaces in contact with the ends of each spring assembly can be limited by limiting the angular travel between the clutch input and output members.
A torsional vibration damper with a plurality of compression springs loaded together in parallel provides limited angular travel of the torque input member relative to the torque output member. To obtain the torsional vibration dampening desired in some vehicle drive trains, the angular movement of the torque output member relative to the torque output member must be increased.
The angular movement between the torque input and the torque output members of a torsional vibration damper can be increased by placing several springs together in series, as shown in U.S. Pat. No. 4,702,721 to Lamarche. This arrangement requires separators, sometimes referred to as skates, between the springs that are loaded in series. Skates add weight, cost and complexity to a torsional vibration damper. The separators are supported and guided in arcuate clots. The ends of the slots provide a single stop for each group of springs in series. The single stop limits compression of only one spring in each group of three springs in series.
The angular movement between the torque input and the torque output members of a torsional vibration damper can also be increased by placing first stage springs in series with second stage springs that are closer to the axis of rotation of damper than the first stage springs. The first stage springs are located along an outer circle and are compressed in parallel to each other. The second stage springs are located along an inner circle and are compressed in parallel to each other. The first stage is in series with the second stage. A torsional vibration damper with first stage compression springs loaded in series with second stage compression springs is shown in U.S. Pat. No. 4,347,717 to Lamarche. Stops are not provided in the two stage torsional vibration damper disclosed inn the '717 patent.
Coil compression springs can be damaged by being subjected to shock loads when they are compressed beyond design limits. These shock loads can lead to broken springs. Stops which prevent springs from becoming compressed beyond design limits can prevent damaged and broken springs.
Torsional vibration dampers with compression springs in stages and the stages in series with each other have a critical need for stops to protect the springs. One of the stages will almost always reach the end of its travel before the other stage reaches the end of its travel. Two or more stages of compression springs that are loaded in series take substantial space, leaving little space for stops. The two stages of compression springs in the '717 damper take most of the available space.
Rivets or pins which can be used for stops, as shown in the '721 patent, can concentrate the forces in a torsional vibration damper. The concentration of forces in a limited number of areas in a damper requires that those areas be strengthened. Strengthening of such areas increases the weight and cost of a torsional vibration damper.