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Friction and wear in rolling and sliding contacts
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Design (Div.).
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Mechanical products are always expected to perform better, last longer, be more environmentally friendly and preferably cost less. Rolling and sliding contacts are found in many mechanical products, and friction and wear in the contacts have a direct impact on the products’ ability to meet these demands. Friction directly influences efficiency; low friction is often wanted to minimize power loss and fuel consumption. Wear generally shortens the lifetime, leading to more frequent service stops and increased costs. Increased demands on the products also means increased demands on the contacts in the products; contacts must work with higher loads and less friction, and they must last longer. The combination of increased demands and the high levels of sophistication of many products puts the spotlight on the contacts, and it becomes perhaps more important than ever to be able to predict and optimize their tribological performance. Simulation of systems with rolling and sliding contacts is a useful tool to understand the contact conditions and to help optimize their performance.

This thesis is focused on friction and wear of boundary-lubricated, non-conformal rolling and sliding contacts. It presents a 3D brush model for transient friction in rolling and sliding contacts that can handle rough surfaces, varying surface velocities and varying normal load. Friction is simulated in interference mesh gears, cam mechanisms, a system with a roller between two planes and a system with a contact between two discs. Friction is also studied experimentally in interference mesh gears and in the contact between two discs. A wear model based on a generalized form of Archard’s wear law and the single-point observation method is used to simulate wear in the contact between the rocker arm pad and valve bridge in a cam mechanism of a diesel engine.

The results show that the friction model can be used to simulate friction in both motion- controlled and force-controlled systems. The model can be used for both detailed contact studies and studies of the overall behaviour of systems with rolling and sliding contacts.

Simulations and experiments show that the efficiency in interference mesh gears decreases significantly depending on the combination of mesh force pressing the gears together and the load on the output shaft. It is also seen that the torque loss varies heavily during a gear mesh depending on the position of the gear teeth and the number of contact points. The results from the simulations are consistent with the experimental results.

The simulations of the cam-roller contact show that the creep in the contact is low except at high cam speeds when there is a period with high creep when the contact is close to the tip of the camshaft. The simulations of the rocker arm pad and valve bridge show that the contact radii of the wear pad and the position of the centre of the wear pad radii have a strong influence on the amount of wear. The simulations also show that the change of surface shapes due to wear can worsen contact conditions with high normal pressures.

The simulations and experiments of force-controlled systems show that contacts can have a strong influence on a system’s behaviour. The contact acts as a spring damper system and can cause oscillations of the system. Simulations show that the oscillations could, at least in part, be explained by the surface roughness. Simulations also show that the creep in the contact is influenced by the contact stiffness and that the contact stiffness is lower for rough surfaces than for smooth. The experiments also show that the creep is higher for a lubricated contact than a dry contact.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , 45 p.
Series
Trita-MMK, ISSN 1400-1179 ; 2005:20
Keyword [en]
tribology, friction, wear, rolling and sliding contacts
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-434OAI: oai:DiVA.org:kth-434DiVA: diva2:11877
Public defence
2005-10-12, B1, Brinellvägen 23, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20101008Available from: 2005-09-28 Created: 2005-09-28 Last updated: 2010-10-08Bibliographically approved
List of papers
1. Friction in spur gears with interference
Open this publication in new window or tab >>Friction in spur gears with interference
2003 (English)Report (Other academic)
Publisher
iii, 12 p.
Series
Trita-MMK, ISSN 1400-1179 ; 2003:16
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-7001 (URN)KTH/MMK/R_03/16-SE (ISRN)
Note
QC 20101008Available from: 2005-09-28 Created: 2005-09-28 Last updated: 2010-10-08Bibliographically approved
2. Friction in spur gears and cam mechanisms using a brush model
Open this publication in new window or tab >>Friction in spur gears and cam mechanisms using a brush model
2001 (English)In: Proceedings of OST-01, 2001, 195-205 p.Conference paper, Published paper (Refereed)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-7002 (URN)
Conference
Proceedings of OST-01, Symposium on Machine Design , Tallin, Estonia, 2001
Note

QC 20101008

Available from: 2005-09-28 Created: 2005-09-28 Last updated: 2012-09-20Bibliographically approved
3. Torque loss in spur gears with interference
Open this publication in new window or tab >>Torque loss in spur gears with interference
2003 (English)In: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, Vol. 217, 385-395 p.Article in journal (Refereed) Published
Abstract [en]

Gears with interference (zero backlash) are used in robots and other precision equipment. The reason for using interference gearing is to improve positioning and motion control. It is then also desirable to compensate for the friction losses and therefore it is important to model the friction losses accurately. In this paper, results from a model for simulating spur gears with interference are compared with results from a test rig for spur gears with interference. FZG gears are used both in the test rig and in the simulations. Brief descriptions of the test rig and the simulation model are given in the paper; more thorough descriptions can be found in studies by Hedstrom and Karhammar and by Spiegelberg, Andersson and Sellgren respectively. There is also a description of the measurements and the procedure of making them. The aim of this paper is to compare the results from the test rig with the results from the simulation model and to analyse the results from the test rig.

The results show that the simulated torque loss corresponds well with the measured torque loss within the range of the test rig. The results from both the test rig and the simulation model show that there are large variations in the frictional torque during one mesh of a gear tooth. The loss increases with the number of contact points and the lowest loss is found when a driving gear flank passes the pitch point. The overall efficiency decreases when the mesh force increases and the lowest efficiency readings are found when a combination of high mesh force and low load is applied.

Keyword
spur gears, interference, test rig, simulation, results, torque loss
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-7003 (URN)000186679100004 ()
Note
QC 20101008Available from: 2005-09-28 Created: 2005-09-28 Last updated: 2010-10-08Bibliographically approved
4. Simulation of friction and wear in the contact between the valve bridge and rocker arm pad in a cam mechanism
Open this publication in new window or tab >>Simulation of friction and wear in the contact between the valve bridge and rocker arm pad in a cam mechanism
2006 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 261, no 1, 58-67 p.Article in journal (Refereed) Published
Abstract [en]

In this paper the surface velocities obtained from a rigid body model are used to simulate friction and wear in the contact between the rocker arm pad and valve bridge in the cam mechanism of a diesel engine. The friction is simulated with two different friction models, a 3D brush model capable of handling transient conditions such as an varying normal load and varying surface velocities and a Coulombian friction model.The wear simulations are based on a generalised form of Archard's wear model. The results presented here show that both the maximum wear depths and the wear distributions are influenced significantly by the combination of wear pad radius and the position of the wear pad radius centre relative to the rocker arm hearing centre. A combination with wear pad radius of 20 mm and centre position of 5 mm is found to give the least wear depths on both the wear pad and the valve bridge. It is also seen that the contact between the wear pad and the valve bridge is mainly a sliding contact and that the transitions from sliding in one direction to the opposite are very rapid. The change of the surface shapes due to wear has a negative effect on the contact situation causing very high contact pressures.

Keyword
simulation, friction, wear, cam-mechanism, brush model
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-7004 (URN)10.1016/j.wear.2005.09.022 (DOI)000238856200010 ()2-s2.0-33745131020 (Scopus ID)
Note
QC 20101008. Conference: 11th Nordic Symposium on Tribology. NORWAY. JUN 01-03, 2004Available from: 2005-09-28 Created: 2005-09-28 Last updated: 2011-09-27Bibliographically approved

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