WEDGELOCK THERMAL & CLAMPING FORCE LIFECYCLE TEST METHODOLOGY
Each SolidWedge is tested under a 100 Cycle Test consisting of four thermal tests and 96 stress analysis (clamping force) tests. Below consists of how WaveTherm goes about conducting these test.
Thermal Test Methodology
Thermal testing is done to determine the total thermal resistance across the contact surfaces of a test plate and wedge lock to a cold wall.
There is a measurable temperature drop when thermal energy flows across the contact area of two surfaces. By measuring the temperature differential between the two surfaces and knowing the thermal energy being inserted into the test plate, a calculation of the thermal resistance of the joint can be calculated.
Parallel Resistance Theory
There are three ways in which the heat from the test plate can be dissipated. Of the three there are two parallel paths to consider in calculating the total thermal resistance of the test plate and wedge lock system:
Terminology
The following terms are used throughout this test procedure:
In order to prevent the heat frame under test from contacting the “bottom” of the channel and thus providing a third surface to transfer thermal energy, there is a spacer approximately 0.5 inches wide and approximately 0.040 inches thick made of a non-thermally conductive plastic such as ABS, or similar, and placed at the base or non-contact side of the cold wall slot. This spacer is removed after the test plate is locked in place.
𝑇𝑒𝑠𝑡 𝑃𝑙𝑎𝑡𝑒 𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 = 𝐶𝑜𝑙𝑑 𝑊𝑎𝑙𝑙 𝑆𝑙𝑜𝑡 𝐻𝑒𝑖𝑔ℎ𝑡 − 𝑊𝑒𝑑𝑔𝑒𝑙𝑜𝑐𝑘 𝐻𝑒𝑖𝑔ℎ𝑡 − 𝑊𝑒𝑑𝑔𝑒𝑙𝑜𝑐𝑘 𝑁𝑜𝑚𝑖𝑛𝑎𝑙 𝐸𝑥𝑝𝑎𝑛𝑠𝑖𝑜𝑛
Here is an example calculation using a standard VITA 48 cold wall with a 0.225” tall wedgelock which has a nominal expansion of 0.025”:
𝑇𝑒𝑠𝑡 𝑃𝑙𝑎𝑡𝑒 𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 = 0.525 − 0.225 − 0.025 = 0.275𝑖𝑛
Material for both the cold wall and the test plates is 6061-T6 aluminum with a surface finish specified as 16 µin (RMS) in the areas of contact to the wedge lock and between the test plate and the cold wall.
- Test Plate (TP) - The test plate temperature is determined by averaging four thermocouple readings. The thermocouple locations is evenly spaced along the length of the test plate, at least an inch (1.0”) from either end. The thermocouples is located between the heat source and the wedge lock, as close to the wedge lock as possible (typically 0.100” – 0.200” from the center of the thermocouple hole to the wedge lock edge).
- Cold Wall Frame Side (TCWF) - The cold wall frame side temperature is determined by averaging two thermocouple readings. The thermocouple locations is centered along the length of the cold wall, at least an inch (1.0”) from either end. The thermocouples is as close to the frame-cold wall interface as possible (typically 0.100” – 0.200” from the center of the thermocouple hole to the specified interface).
- Cold Wall Wedge Side (TCWW) - The cold wall wedge side temperature is determined by averaging two thermocouple readings. The thermocouple locations is centered along the length of the cold wall, at least an inch (1.0”) from either end. The thermocouples is as close to the wedge lock-cold wall interface as possible (typically 0.100” – 0.200” from the center of the thermocouple hole to the specified interface).
To calculate the total resistance of the system, each path’s thermal resistance is first calculated separately. The total resistance of the system can then be calculated using the same method as parallel resistors in an electrical circuit:
- Frame-side Thermal Resistance (RF) - The thermal resistance from the test plate to the frame side of the cold wall can be calculated as the temperature difference between the test plate and the cold wall frame side, divided by the amount of power dissipated through that path.
- Wedge-side Thermal Resistance (RW) -The thermal resistance from the test plate to the wedge side of the cold wall can be calculated as the temperature difference between the test plate and the cold wall wedge side, divided by the amount of power dissipated through that path.
- Total Thermal Resistance (RT) -The total thermal resistance of the two parallel paths can be calculated the same way as two parallel resistors in a circuit.
Power Levels
Clamping Force Test
Abstract
Clamping force testing is performed on the WaveTherm SolidWedge as well as other various wedgelocks to compare the output force generated by a given input torque. The wedgelocks are secured onto their corresponding mounting plates and the mounting plate will be inserted into the clamping force test assembly. Load cells in the assembly are used to read the clamping force of the device at a specified torque. There is a specific software that pairs with the data reader utilizes (OM-DAQ-USB-2401, Omega Data Acquisition Module) that graphs the pounds applied to the load cells of the test fixture as the test commences.
Test Setup
The test setup requires a test plate with the correct dimensions correlated to the size of the test specimen and the test fixture as seen below. When combined the test specimen and test plate need to be .525”. The width of the testing fixture with the standard load transfer block is .600”. The clamp force test fixture is meant to simulate a real chassis. The torque wrench is used to mimic real world application of a SolidWedge applied within an embedded system. Each specimen should have a cycle of 2-24, 26-49, and 51-99 clamp force cycles for a total of 96 cycles. The 1st, 25th, 50th and 100th cycles are the thermal tests. All data for each test is collected within separate excel files and then consolidated into one overall spreadsheet.
Figure 3: Stress Analysis Fixture