PUMP RATE CHANGE VS. RUN TIME OF A RADIAL SHAFT SEAL

Transcription

1 PUMP RATE CHANGE VS. RUN TIME OF A RADIAL SHAFT SEAL Mark Reimer, Maria Mendizabal ESP International ARTICLE INFO 13 August 2012 ABSTRACT In this study, the pump rate of the seal lip was measured over a period of time to learn how the pump rate of the seal lip modifies over time. Introduction The challenge of sealing against a dynamic surface has been around since the frontier era. The first known seals were leather straps used to retain animal fat on the end of a wheel axel. This crude seal required routine maintenance. The Industrial Revolution spawned the development of engines, transmissions and gearboxes. The seals of the industrial age were organic ropes or packing. These seals were very effective until speeds, temperature and other parameters increased with the development of better transportation systems. In the late 1920 s, a self contained shaft seal was created from oil resistant leather assembled into a metal case. This was the first radial lip seal to take advantage of a metal press fit as an outside diameter seal. After that, radial shaft seals develop; a synthetic, oil-resistant rubber replaced the leather element, forever changing seal design. Radial shaft seals prevent leakage through the generation of a pumping action at the interface of the seal lip and the shaft surface, the pumping direction is a direct correlation to the direction of an asymmetrical contact pressure profile (See Figure 1). This pressure profile is controlled by the geometric design of the lip seal which is designed to create a larger pressure gradient on the oil side of the sealing lip. This pressure gradient is one aspect that contributes to the function of a radial shaft lip seal. A second aspect that contributes to the pumping mechanism of a seal lip is the presence of an oil film layer between the seal surface and the shaft surface. Jagger [1] [2] successfully demonstrated the existence of this oil film layer between the shaft and lip. Figure 1: Dynamic Sealing Mechanism

2 The third aspect in the sealing mechanism that contributes to the net pumping affect of the seal lip is the formation of asperities on the seal lip surface. As shown by Müller [3], the asperities become aligned at an angle to the rotating shaft causing the oil film to pump towards the oil side of the seal. The existence of the pressure asymmetric pressure profile, fluid film and asperities all contribute to the pumping mechanism of the seal. It is our opinion that the life of a seal lip is modified when one or more of these items are modified. In this study, the affect of lip wear on the seal lip pump rate is investigated. As the seal wears the contact width of the seal lip grows and the asymmetric pressure profile is modified. As well, the elastomer of the seal will harden over time due to its exposure to fluid at temp. Eventually both factors contribute to the occurrence of a seal leak. In this study, we have run a lip seal and measured the change in pump rate over the life of the seal. This is done to demonstrate the loss of pump rate as the seal lip wears and that loss of pump rate will result in a seal leak. Test Seal For this study, a specific seal size was chosen. The seal used in this test was ESP part number TBC FCS. Figure 3 shows the profile of the seal and Table 1 Figure 3: Helix Style displays the designed bore and shaft sizing for the test seal. To aid in the pumping measurement, the two dust lips shown were removed. The test seal had a bidirectional U style helix (as shown in Figure 2). The helix design parameters are displayed in Table 2. The test seal has a Fluorocarbon lip with a stainless steel garter spring. The seal lip design is a common design widely used in other seals. Figure 2: Test seal profile ID OD Width Lip Material mm mm 8.00 mm Fluorocarbon Table 1: Seal Parameters Aid Style U Rib Spacing 2 Rib Height 0.05 Helix Angle 30 Number Protrusions 6 Table 2: Helix Parameters

3 Pumping Rate Test Method A pump rate measurement method similar to the method used in Yang [4] was used in this study. The equipment used is depicted in Figure 4. Here the test seal is placed on the shaft between the internal and external cavity with the oil side of the seal lip towards the internal cavity. The test seal was run for 50 hours for the break in period at 5000 RPM. The shaft diameter was mm (0.984 ), the bore diameter 50.0 mm (1.85 ). To measure the pump rate the internal cavity (Figure 4) had a port at the top where the oil pumped by the seal would exit and would be collected for measurement After the break in period the seal ran for 100 hours at 5000 RPM, in John Deere J20C hydraulic oil, at a half shaft level and heated to 110 C. After each 100 hours the exterior and interior cavity was filled with oil, the seal was then run at the desired measurement speed for 30 minutes in order to attain a stable measurement. After that the speed was changed to 2000 RPM and the process was repeat until the max speed of RPM. Figure 4: Pump Rate Testing Machine

4 Results The graph in Figure 5 shows the data collected during the test. As shown in the data, the pump rate was very high initially then started to reduce over time. This seal was tested until a leak occurred at the 5000 RPM shaft speed. TBC FCS Pump Rate g/min RPM Figure 5: Pump rate vs. Time vs. Shaft speed Conclusions The seal lip pump rate performed as expected and reduced as the seal lip wore. The one thing that we did not do in this study was monitor the seal lip contact width at each 100 hour interval. With that added data we could demonstrate that there is a direct correlation between seal lip pump rate and contact width growth. The other thing that we did not do was monitor the change in elastomer compression set. We are planning a second seal test that will include this information.

5 Works Cited [1] E. T. Jagger, "Rotary shaft seals: the sealing mechanism of synthetic rubber lip seals running at atmospheric pressure," Proceedings of the IMechE, vol. 171, pp , [2] E. Jagger, "Study of the lubrication of synthetic rubber rotary shaft seals," Proceedings of the IMechE Conference on Lubrication and Wear, pp , [3] H. Müller, "Concepts of sealing mechanism of rubber lip type rotary shaft seals," in Proceedings of 12th International Sealing Conference, [4] A.-S. Yang, "Parametric study of helix configuration in ribbed lip seal," Tribology International, vol. 53, pp , 2012.