Patent Application
20190366503
The device relates to a crankshaft grinding machine having grinding wheels which are supported by an overhead beam and have a pendulum motion to follow the crankshaft pins as the crankshaft is rotated about its longitudinal axis.
In a typical prior art crankshaft grinder, the crankshaft is mounted between a headstock and a tailstock or between two headstocks, with one or more grinding wheels being used to grind the pin bearings on the crankshaft. The grinding wheel is normally mounted on the side of the crankshaft, and because of the orbital motion of the pins relative to the main bearings of the crankshaft, the grinding wheel has to move in and out relative to the crankshaft in order to accurately create the surface of the pin that is being ground. The grinding wheel and the work centerline in these designs exist on a common plane that is essentially horizontal. The grinding wheel spindle is normally supported on a hydrostatic linear bearing so that it can be moved in and out relative to the crankshaft smoothly, and with the least amount of friction. Such a machine arrangement may be called planar or Cartesian.
For reasons of economy, it would be desirable to design a crankshaft grinding machine which requires less floor space than conventional planar or Cartesian crankshaft grinding machines. It would further be desirable to design a high efficiency crankshaft grinding machine that requires less power to move the grinding wheel in and out relative to the crankshaft, and that does not utilize a hydrostatic linear bearing which requires lubrication to support the grinding wheel feed axis.
An upright crankshaft grinding machine has a smaller footprint than a Cartesian grinding machine and includes an overhead beam which supports a crankshaft and crankpin grinding wheels. Cartesian grinding machines typically use linear hydrostatic bearings positioned below the spindles of the grinding machine. During the machining process, Cartesian grinding machines can create debris in response to creating the surface of a crankshaft. This debris can infiltrate the fluid used by the linear hydrostatic bearing and interfere with optimal operation of bearing. In addition, heat generated during the machining process may impinge on the Cartesian grinding machine hydrostatic bearing, machine base, and other structure thereby changing its geometry and decreasing the precision with which crankshaft surfaces can be created. In contrast, an upright crankshaft grinding machine is not reliant on hydrostatic bearings and can use roller bearings, which are less susceptible to interference from debris due to their location above the working zone. In addition, the upright crankshaft grinding machine suspends grinding wheels from a pivot axis, and are part of a pendulum assembly which has a natural oscillation frequency. Controlling the pendulum assembly to pivot with a pendulum motion back and forth at a natural oscillation frequency reduces the energy required to move the grinding wheels in and out to follow the pins of the rotating crankshaft.
Turning now to the figures,
A headstock carriage 25 may be mounted on the horizontal beam 21 in the machining space 22. Headstock carriage ways 27 may run along the length of the horizontal beam 21 and may be used for coupling the headstock carriage 25 to the horizontal beam 21, whereby the position of the headstock carriage 25 along the horizontal beam 21 may be adjusted.
A tailstock carriage 26 may be mounted on the overhead beam 21 in the machining space 22. Tailstock carriage ways 28 may run along the length of the horizontal beam 21 and may be used for coupling the tailstock carriage 26 to the overhead beam, whereby the position of the tailstock carriage 26 along the horizontal beam 21 may be adjusted. The tailstock carriage ways 28 may be extensions of the headstock carriage ways 27, or the headstock and tailstock carriage ways may be independent from one another according to the machine design. A workpiece 12 such as a crankshaft to be ground may be mounted between the headstock carriage 25 and the tailstock carriage 26. Grinding wheels 30 may be positioned in the machining space 22 so that they contact the pins on the crankshaft 12 as described more fully below.
The pivot drive 38 may comprise a coaxial torque motor 44 and precision axial encoder 45 positioned in the upper pivot end 40 of the arm 37 which supports the grinding wheel 30. The coaxial torque motor 44 and the precision axial encoder 45 can be incorporated into the upper pivot end 41 such that the rotational output of the motor 44 is coaxial with the pivot axis 42. That is, an output or output shaft of the coaxial torque motor 44 is coaxial to the pivot axis 42. The precision axial encoder 45 can be mounted in relation to the upper pivot end 41 such that it monitors the angular position of the arm 37 as it rotates about the pivot axis 42 in response to the output of the coaxial torque motor 44. In one implementation, the coaxial torque motor 44 can be implemented using a BoschRexroth MBT201C-0010-F torque motor and the precision axial encoder 45 can be implemented using a Heidenhain RCE 8510 encoder. This torque motor can have a 200 Nm peak capability. The particular combination of torque motor and encoder identified above can yield a 1 arc-second accuracy and 32,678 sinewaves per revolution that can further be interpolated by a CNC processor by as much as 32,678 to provide angular resolution of over 1 billion count per revolution (230). If the arm 37 has a length of 250 mm, 1 arc second of accuracy at the pivot axis 42 results in 1.2 μm of error tolerance on a crankpin of the crankshaft.
The coaxial torque motor 44 can be used to rock the arm 37 back and forth allowing the grinding wheel 30 to follow the pin of the crankshaft 12 to create a finished surface moving in an orbital motion around the axis of the crankshaft 12. The precision axial encoder 45 may be used to control the rotation of the torque motor 44 which creates the back and forth rocking of the grinding wheel 30.
A grinding wheel spindle 49 may be mounted on the lower end 41 of the arm 37 and is positioned in the machining space 22. The grinding wheel 30 may be driven by the grinding wheel spindle 49. The grinding wheel 30 has an axis of rotation 51 that is parallel to the horizontal beam 12. The grinding wheel spindle 49, the grinding wheel 30, and the arm 37 forms a pendulum assembly 52 that has a center of mass 53 that is spaced from the pivot axis 42 of the pivot drive 38. The pendulum assembly 52 may be driven to have a pendulum motion at a natural oscillation frequency about its resting or equilibrium position. A pendulum motion is defined by motion of the pendulum an equal distance to either side of the resting or equilibrium position of the pendulum. The natural oscillation frequency of a pendulum is determined by the length of pendulum measured from its pivot axis to the center of mass of the pendulum assembly.
A trough 57 may be positioned at the base of the rectangular frame 16 for catching debris from a machining operation. The trough 57 may be positioned below the elongated workpiece 12 and the grinding wheel 30. By positioning the grinding wheel carriage ways 35 and the headstock and tailstock carriage ways 27, 28 above the elongated workpiece 12, debris from a grinding operation is precluded from falling into and fouling the ways 27, 28, and 35.
In operation, the pivot drive 38 may be used to rock the arm and the grinding wheel 30 back and forth in a controlled manner about the pivot axis 42, to ensure the pendulum assembly 52 is displaced equal angular amounts D1 and D2 in opposite directions from its equilibrium position 55. Rocking the pendulum back and forth in this way minimizes the amount of power required to displace the grinding wheel 30 in order to follow the path of the crankshaft pin 56.
Having thus described the device, various modifications and alterations will occur to those skilled in the art, which modifications and alterations will be within the scope of the appended claims.
