Throughout time, many test setups have been developed to evaluate the behaviour of cam-roller interfaces and improve their tribological performance. However, the working conditions of these tribological systems are often not accurately captured. The work “Tribological Testing of Cam-Roller Follower Systems (Design of the CRT-01)” delivers a novel testing method and the design and construction of a novel test setup, together with a strategy to evaluate cam-roller follower systems under real life conditions. A summary of this work is presented next.
Heavily loaded cam-roller follower mechanisms are part of diesel injection systems used in truck diesel engines. Their main function is to convert rotary motion into linear motion that can be used to generate fuel pressure (Fig.1). During the generation of fuel injection pressure, the load acting at the cam-roller contact can be as high as the weight of a compact car (~12kN).
Fig.1. Heavily loaded cam-roller follower mechanism.
Cam-roller followers can be divided into 3 components, the cam, the roller, and the pin, hence leading to two interfaces (i.e., tribological contacts), the cam-roller interface, and the pin-roller interface (Fig.2). The cam roller contact is highly dynamic, the conditions at the interface are constantly changing in a brusque fashion. The contact load changes, the surface speed changes, the roughness around the cam changes, and the radii changes during every revolution. Additionally, the interaction and interdependence of two tribological contacts (i.e., the pin-roller contact and the cam-roller contact) increase the complexity of this mechanism from a tribological point of view. The frictional torque generated at the pin-roller interface combined with the rotational inertia of the roller generate a resisting torque (τ) that can in some cases slow down the speed of the roller leading to slippage. Roller slippage must be avoided or minimized to reduce wear and friction losses. Due to the complex tribological behaviour of cam-roller systems, ensuring high reliability and optimum lubrication poses a difficult engineering challenge. To ensure optimum lubrication and high reliability, the performance of cam-roller interfaces must be evaluated throughout tribological tests.
Fig.2. Cam-roller follower components and interfaces.
The testing method and the novel test setup developed enable the measurement of critical parameters affecting the tribological performance of cam-roller interfaces (e.g., slip and traction). At the same time, accelerated testing strategies are incorporated. Next, the development of the concepts and the realization are discussed.
Generation of Concepts and Validation
The selected concepts include a “winner” testing method concept and a “winner” test setup concept. The testing method describes what can be done to test cam-roller interfaces whereas the test setup concept explains how it can be done.
The main goal behind the proposed testing method is to mimic real-life conditions of cam-roller interfaces in a simple, efficient, and reliable way. In a real-life scenario, a certain load (l), speed (s), and resisting torque (τ) can be expected in the different regions around the cam, e.g., flanks, nose, base circle (see Fig.5) when they get in contact with the roller. A testing strategy would be to mimic the specific cam operating conditions on a “round” cam, i.e., varying the specific load (l), speed (s), and resisting torque (τ) around the circumference as shown in Fig.5.
Fig.5. Strategy for testing cam conditions on a “round” cam.
The proposed testing method concept can be further described by presenting the following five design decisions shown in Fig.6.
- Replace the real cam with a “round” cam for testing. This simplification is acceptable since it is of particular interest to test the cam-roller mechanisms at low speeds (i.e., below 300 RPM) where the effects of inertia are negligible. In that way, the load, the speed, the roughness, and the radii can be kept constant.
- Use an actual roller to form a heavily loaded line contact.
- Replace the pin-roller contact with needle bearings and fix the roller to a shaft to enable speed measurement.
- Form an oil bath to partially submerge the “round” cam and drag lubricant into the interface during rotation.
- Apply a controlled resisting torque (τ) resembling the pin-roller frictional moment and generate (non-controlled) slippage.
Fig.6. Embodiment of the testing method concept.
The proposed test setup concept features 3 design solutions to 3 design problems, namely, the incorporation of a self-alignment system to effectively operate with a line contact, the use of a flexure-based system to enable the measurement of traction forces with negligible dissipation, and the generation of roller slippage through the application of a resisting torque to the shaft of the roller. The proposed solutions have been validated with the proof-of-concept prototype shown in Fig. 7 (see https://www.youtube.com/watch?v=1TWosF5WFec).
Fig.7. CRT-01 Proof-of-concept prototype.
Detail Design & Development
The successful concept validation gives green light to move on to the Detail Design & Development. During this phase, the concept is refined until a final design is achieved. Fig.8 shows the detailed design of the CRT-01 and its 5 constituent systems, namely, the Tribological System (T), the Loading System (L), the Braking System (B), the Rotation System (R), and the Data Acquisition & Control System (D).
- T is the core of the setup, and it is where the tribological tests take place.
- L allows the application of high loads on the contact.
- B enables the application of a controlled resisting torque generated by a magnetic hysteresis brake.
- R drives the round cam.
- D allows the acquisition of signals and data logging.
Fig.8. CRT-01 detailed design.
The CRT-01 can be further divided into subsystems. Taking the Tribological System as an example, it can be furthered partitioned into the Roller-Alignment System, the Traction Force Measurement System, the Lubrication System, and the Heating System as shown in Fig.9. The assembly of the CRT-01 into its components can be seen in the following animation: https://www.youtube.com/watch?v=LViW5rxueoM
Fig.9. Subsystems of the Tribological System.
Several custom-designed parts and off-the-shelf components have been manufactured and purchased, respectively to put together an assembly of approximately 500 parts. Fig.10 shows the specimens mounted in their respective housings. Fig.11 shows the round cam and the roller mounted in the test setup to run a test.
Fig.10. Roller and round cam housing.
Fig.11. Roller and round cam mounted for testing.
Fig.12. Real-life CRT-01.
To evaluate the repeatability of the CRT-01, three tests were performed under the same conditions. For the tests, the loading and speed conditions were fixed and the resisting torque (i.e., load torque) was gradually increased to generate (and measure) slippage. At the same time, the traction force was measured to calculate the traction coefficient. Fig.13. shows the slide-to-roll ratio (SRR) as a function of the load torque. For load torques below 7 Nm, the lines overlap. However, at higher torques, the amount of slip produced during the second test is higher than that of the first test and the slip produced during the third test is higher than that of the second test. The rise of slip from test to test can be attributed to the evolution of the round cam surface produced by polishing wear, i.e., the tractive behaviour of the surface changes throughout the tests. After each test, the surface of the round cam becomes more slippery due to the loss of roughness leading to increased SRR levels.
Fig.13. Load torque vs. slide-to-roll ratio (SRR).
Fig.14. shows the traction coefficient as a function of the load torque. The results obtained during the 3 tests overlap showing that the traction force measurement system yield to repeatable results. Interestingly, even though the amount of slip generated at high load torques varies from test to test, the traction coefficient remains the same. This can be explained by the evolution of traction curves (Fig.15). Fig.15 shows 3 different traction curves. During Test I, the round cam roughness provides good traction, and hence, high traction coefficients can be attained at low SRR levels. During the second test, some polishing has occurred affecting the tractive properties of the surface. Consequently, higher SRR levels are required to attain the same traction coefficients from the previous test. During Test III, the surface roughness has been significantly removed and it has become much more slippery. Hence, the amount of slip required to attain the same traction coefficients from previous tests is even higher. The amount of polishing wear produced after the tests and the evidence of excellent alignment can be seen in Fig.16.
Fig.14. Load torque vs. Traction coefficient.
Fig.15. Evolution of traction curves throughout testing.
Fig.16. Wear scar on the round cam.
A robust testing method, a specialized test rig and a strategy to virtually test the influence of pin-roller friction on the tribological behaviour of the cam-roller contacts have been developed. The CRT-01 exhibits a precise cam-roller alignment leading to a reliable performance. The braking torques applied generate sufficient slip making it possible to effectively measure the traction forces with negligible dissipation.