Description Of The Hohmana9 Wear Simulator

The simulator incorporates a complete piston ring secured in a disk shaped holder. The ring ends are held together by pins and a clamp to a nominal gap clearance. This unit reciprocates with a fixed stroke of one inch, and portions of the ring are loaded against two liner segments which are placed 180° apart. Each stationary liner segment is about 1.5 in long and 0.27 in wide, and is cut from the top of a finished liner. The ring holder is powered by a 1 hp d.c. electric motor that is speed controlled by a SCR type controller. Load is applied to the ring liner interface by means of a bellows and lever arm arrangement. Provision is made for a lubricant spray at the wear interface and the entire unit may be heated to about 1.000 F to simulate top ring reversal conditions. Data obtained from the Hohman-A9 simulator include friction force as a function of time, wear volume, and surface finish.

Figure 1 shows a schematic of the simulator. By pressurizing the bellows, ring loads are applied to simulate the high pressures normally experienced in highly turbocharged engines under HDD conditions. . At present, maximum reciprocating speed is 1000 rpm. The reciprocating force is transduced by a strain gauge load cell whose lead wires are routed to an amplifier by means of a Grasshopper linkage.

The strain gauges report the friction force between the liner segments and ring, the friction force in the reciprocating sleeve bearing, and the inertia force.

As required, lubrication is provided by means of stainless steel tubes through which a pressurized air and oil mixture pass. The oil is provided as a spray mist directed to the rubbing surfaces. A peristaltic pump, capable of precise, variable feed rate, is used to control oil quantity. A double wall, insulated, stainless steel oven surrounds the liner samples and ring. Electric heaters are used to produce oven temperatures exceeding 1000 F. Heater control is by a Chromalox controller which cycles the heaters on and off. Thermocouples are employed at several points within the oven to indicate temperature. A 60 tooth gear and magnetic pickup are used to trigger the computer to sample the strain gauge force. Since the force measured includes that of inertia as well as reciprocating shaft bearing friction, these forces must be determined separately and subtracted. This is done in a separate test in which the bellows force is eliminated (Keeping the samples in place) while the tester is run at the required speed. The unwanted forces are thus determined and then subtracted, leaving as a result the friction force between the ring and the liner segments as a function of crank angle.

The friction coefficient is determined by dividing friction force by normal load provided by the bellows. Friction coefficient is calculated and stored every 6 degrees of crank rotation. The friction coefficient is also averaged over one revolution. Friction data is displayed on the monitor screen and recorded on floppy disk. Upon completion of the test, the ring and liner profiles are measured to determine wear and surface roughness. Liner and ring roughness profiles are taken in the axial direction of the cylinder. Ring and liner surfaces are examined and photographed by light microscopy and by scanning electron microscopy to determine mechanisms of wear and failure.

For process control, as well as data acquisition and analysis, a computer system is employed. Speed, load and test temperature input are provided as a function of time and serve as the process control function. As presently constructed, the Hohman-A9 tester is capable of unattended operation for a prescribed amount of time over a preset cycle. Automatic shut-down occurs if variables exceed preset limits. Periodically, friction data are taken, displayed on the screen, and stored on floppy disks. Table I gives a list of the computer controlled features.

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