Robert Cep, Jan Strbka, Lenka Cepova
Department of Machining and Assembly, Faculty of Mechanical Engineering,
VSB – Technical University of Ostrava, 17. listopadu 15/2172, 708 33 Ostrava, Czech Republic
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Abstract:This paper is about determination of change of roughness character under rotary shaft packing for increase of operation life of packing point at preservation of packing property.
A part of paper is comparison of roughness sizes of shaft at centre grinding and determination of optimal characteristic.
Key words: Roughness, Packing, Grinding, Wearing, Friction.
1Name of the author, title, company, address and e-mail.
2Name of the author, title, company, address and e-mail.
- INTRODUCTION
The functional properties of parts and machine surfaces depend in alarge extent on the surface character. Optimum choice and observance of surface character requirements during product evaluation affects surface performance characteristics, liability and working life of frames and mechanism, including environment effect resistance [1].
Similarly is it with the surface character of the shaft in the sealing bushing, where the shaft and sealing bushing edge active area prevent the external environment impacts on functional parts of the equipment.
- PROBLEM DEFINITION
The issue of the surface under the sealing bushing and sealing bushing working life is very complex. Determination of the optimum solution of the issue between these two parts is difficult, because there interact different tribological factors that by some means depend on each other. See fig.1.
From the point of view of tribology on the sealing system, the friction and sealing working life are in the sealing system interconnected. The mutual influence of these properties depends on the parameters interaction (temperature, pressure and velocity).
Since this issue is quite extensive, we focused in this article on the determination of the optimum shaft surface profile.
The sealing effectwhich should ensure the sealing bushing basic required parameters and the quality requirement of shaft surface as well as lubrication oil.
The friction effect of two adjacent sliding surfaces is minimalized if the two friction surfaces are separated by lubricant layer.
The lubrication oil is used for the friction reduction. This lubricant must be able to resist various conditions e.g. ambient temperature, feed rate, normal force.The sealing bushing wearing depends extensively on the shaft surface profile.
Fig.1. Factors incoming into the sealing process
Fig.2. Friction types
2.1.Surface quality
The evaluation of the sealing mechanism surface quality depends on the surface profile and values Rz, Ra.The shaft surface structure is effected by the technological method of processing during finishing.
For reaching of the high sealing effect it is necessary to provide the most optimum size of roughness with the lubrication oil. The peaks of Rz must be as periodical as possible.
However this is by the recessing method difficult to achieve.Using the recessing method on the shaft surface generates the same surface profile as on the grinding wheel.
For the sealing effect it is necessary that there isn’t any line on the shaft surface after the machining. If there such line appears e.g. in the direction out of the device, it would occur lubrication oil discharge. On the other hand the line towards the device would cause the supply of impurities from the environment and the inpurities would get into the contact with device and result in damage of the sealing.
That’s why recess grinding method is used for finishing.
For the shaft guidance control we use ,, thread method´´, where on the shaft with oil film is aline marked with pencil. Weight on thread is then hung on the shaft. (Fig. 5)
The shaft is twisted in one and then in another direction and the thread would in case of the movement on surface wipe the oil film.
That’s why the surface profile depends substantially onthe grinding wheel type, its composition and grain size. The tests proved, that another important aspect for the surface forming is the removed layer thickness (ap.).
Fig.3. Determining lead on the shaft
2.2.Lubrication oil
The lubrication oil creates on the shaft surface the oil film which separates the sealing surface from the shaft surface and reduce the friction.
For this effect it is important to have ceratin dimensions of the shaft surface Ra, Rz. It is due to the fact, that if the surface is too smooth, it would prevent the lubrication oil from making an even oil film (Fig.4).
In such case there would negative operation of sealing occur.
On the surface with created cutting marks (after tool) which are within the limits the conditions are perfect for the lubrication oil to get into the grooves and this way create coherent oil film (Fig.5).
Fig.4. Non-wettable surface
Fig.5. Ground surface
Too rough shaft surface cause the friction against the tops of the profile asperity which then lead to the wearing in the sealing tip contact area and enlargement of the contact area of the sealing and shaft surface. In this area high friction and temperature occur and the functionality and service life of the sealing is significantly decreasing.
- RECOMMENDED VALUES OF THE SEALING SYSTEM FOR THE SHAFT QUALITY
Tolerance ISO h11
Roundness IT 8
Ra = 0,2 to 0,8 µm
Rz = 1 to 4 µm
Material 42CrMoS4
Hardness 45 to 60 HRC
Grinding wheel: 60 to 100
3.1.Testing procedure and testing conditions
In the test of the ideal surface profile we examined the abrasivity, it means the sealing material wearing. For comparison we chose 4 types of sealing materials which were proper for the stated conditions.
PTFE - polytetrafluoroethylene
PTFE C104. carbon fibre alloy
PTFE B602 bronze alloy
PTFE GM201 glass fibre mixture MoS2
By achieved roughness Ra = 0,3 µm.
For the achieving of the required roughness we used grinding wheels of Winterthur producer marked 57A80J7V , 57A60J7V and 57A100J7V
3.2.Test parameters
Wheel speed 45 m.s-1
Complete spark out.
Mesh depth 0,03mm
Recess method.
Shaft material 42CrMoS4, high-frequency hardened for hardness 53+7 HRC
Sealing internal diameter63,400 mm
3.3.Test procedure
The shaft surface was machined using the grinding wheel for the roughness Ra=0,3 µm with the identical cutting parameters.
In the next phase, the wearing on these surfaces was examined by inspected sealings. The wearing took place in the real conditions where the given shaft was used and that was in hydraulic device, where we used the shaft with sealing and loaded for 50 % using 10000 cycles.
1 cycle- start up- pressure 14 bar.- shutdown, start up in opposite direction -pressure 14 bar- shutdown.
After finishing the load test the change of the sealing internal diameter size using 3D measuring device Mitutoyo was measured. From this test two best materials of the sealing (subjected to the load test with testing time 336 hours with 60 % load andpressure 5 bar) were chosen.
- MEASURING RESULTS
Table 1. Results for 10000 cycle
Ra=0,3 / PTFE / PTFE B602 / PTFE C104 / PTFE GM 20157A80J7V / 63,309 / 63,379 / 63,372 / 63,314
57A60J7V / 63,378 / 63,394 / 63,393 / 63,385
57A100J7V / 63,295 / 63,341 / 63,338 / 63,301
Fig.6. Results for 10000 cycle
Table 2. Results for 336 hours
Ra 0,3 / PTFE B602 / PTFE C10457A60J7V / 63,358 / 63,343
Fig.7. Results for 336 hours
- CONCLUSIONS
From the obtained values and diagram it is visible, that the minimum wearing reached the sealing using the grinding wheel with the grain size of 60, and the worst with the grain size 100.
For all types of grinding wheels the least wearing got the material with bronze alloy and then material with the carbon fiber alloy.
For the next testthe sealing materials PTFE B602 andPTFE C104 with the surface profile which was left by the grinding wheel 57A60J7V, consequently will be chosen.In this test the best results had the material with bronze alloy.
From the test results suggestion is to use, under stated conditions, the sealing made from PTFE B602 material, because it passed the test with the best results from all tested materials.
REFERENCES
[1]NESLUŠAN M., TUREK S., BRYCHTA J., ČEP R., TABAČEK M. Experimentálne metódy vtrieskovom obrábaní. Žilina : Edis 2007, 311-346s. ISBN 978-80-8070-711-
[2]VASILKO, Karol. Technológia dokončovania povrchov. Prešov : Cofin 2004, 114s. ISBN 80-8073-124-1
[3]BEČKA, Jan. Tribologie. Praha : ČVUT 1997. 211s.ISBN 80-01-01621-8
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[5]SADÍLEK, M.; ČEP R.; GREGUŠOVÁ M. Vliv řezné rychlosti na drsnost povrchu při frézování tvarových ploch. Zborník referátov z 8. medzinárodnej vedeckej konferencie „Nové smery vo výrobných technológiách 2006“, Fakulta výrobných technológií Technickej univerzity v Košiciach so sídlom v Prešove, 2006, s. 109 – 113. Prešov, Slovenská republika. ISBN 80-8073-554-09.