Wheel Root Wear.

          Wheel turning of a Diesel Loco is mostly done due to rapid generation of root wear in one of the wheels of locomotives. Pre-mature wheel turning not only results in increased ineffective, reduced outage but also causes reduction in wheel life. Intra and inter bogie wheel diameter matching requirement causes wheel turning of good unaffected wheels also.

It is important to note that development of root wear is not uniform in all wheels & in all locations clearly indicating presence of local factor(s) causing localized root wear. Wheel turning records clearly indicate that all locos or all wheels do not develop high root wear. Only a few locos at one or two locations exhibit high root wear.

Challenge before maintenance engineer is to identify the factors causing rapid root wear & take corrective measures. Unless root cause of root wear is attended, simply turning of wheel will not solve the problem & again rapid wheel root wear will take place. Factors contributing development of root wear must be understood properly to formulate an effective strategy to arrest root wear in wheels.

          Root wear can develop only when there is excessive wear on root. This excessive wear is contributed due to:

·        Rubbing of wheel root with brake blocks

·        Rubbing of wheel root with track flange.

Each factor has to be tackled separately.

Root wear due to rubbing with brake blocks.

Tell tale signs in such cases are

·     Biased wear on brake blocks: Brake block rubbing on wheel flange due to shifting

·     Asymmetrical placement of brake blocks.

·     Brake block rubbing with root in one side and projected towards tread on other side.

In such cases, during braking, brake block partially mounts/slides over root causing root wear of wheels. Had brake block been correctly applied over tread, root wear would not have taken place.

·  Play in brake hanger

·  Bent tie rod

In such cases, brake hanger brake shoe or complete brake hanger assembly keeps on swaying & during brake application sometimes mounts over wheel root causing root wear.

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Action to be taken:

1.    Ensure correct positioning of brake blocks: MS washer of 4mm thickness must be inserted in the hanger assembly to enable brake head assembly to move away from the flange.

2.    Proper fitment of brake blocks over brake heads should be ensured.

3.    Width of brake block should be measured before fitment. Wider brake blocks cause wear of wheel flange root.

4.    Alignment of brake blocks should be parallel to wheel treads with uniform clearance and without any skewness.

5.    Bent or misaligned tie bar results in brake blocks being pulled towards the flange of the wheel and riding of brake block against wheel root flange causing excessive root wear. Reinforced tie bars should be fitted in M₂₄.

6.    Pool of intermediate size wheel to be kept to obviate need of wheel turning of entire bogey/loco for root wear of one wheel.

             Root wear due to wheel rubbing with track flange

              

Wear occurs from sliding between wheel & rail typically in the flange root area.

Too high wear is cause of concern and should be monitored by monthly wheel gauging.

Wheel Rail dynamics: The wear of wheels and rails results from a complex dynamic relationship within the movement of wheel at the track with wheel spin. The kinematical properties of wheel & rail contact such as rolling radius, contact angles and wheel set roll angle vary as the wheel sets moves laterally relative to the rails.

Because of the curving of the wheels as the leading wheel sets moves outwards, the radius of the outer wheel becomes greater that the inner wheel. As both wheels are rotating at the same speed, the larger radius wheel tries to roll further than the smaller radius wheel.  Wheel set is thus steered towards a radial alignment, and it rolls smoothly around the curve. The opposite process happens on the trailing wheel sets as it moves inwards on the curve. The forces that are generated depend upon the effective conicity of the wheel set on the rail. The larger the conicity, the greater the rolling radius difference for given lateral shift. Conicity tends to increase with increasing wheel tread wear.

In practice, rotation of the wheel sets into radial alignment is resisted by vehicle suspension. The stiffer the primary yaw suspension, the larger the forces which will be required to achieve the required rotation.

Once the wheel set is unable to generate sufficient longitudinal forces to steer into radius position, the wheel sets will have an angle of attack to the track and will run in flange contact. Because of the angle of attack, both of tread contact points will be generating forces to push the wheel set into the flange, which must be resisted by flange contact force. These forces are a major cause of flange wear.

Poor curving vehicles on curvaceous routes will suffer mainly flange wear whereas good curving vehicles on relatively stringent routes will suffer mainly tread wear.

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Wheel wear is influenced by wheel rail contact angle.

How track friendly a vehicle is depends not only on its design, speed & axle load but also on its maintenance condition.

It is not uncommon for wheels of both sides of a wheel axle to degrade differently despite having the same axle load. Bogies’ dynamic curving performance by checking left & right rotation as well as bogies ability to return to a neutral tracking position is important. Leading axle is first to negotiate the curve & therefore usually have larger lateral force.

Condition monitoring of root wear:

A bogie maintenance strategy based on detection of axles that exhibit higher levels of instability (hunting) should be formulated. An exception List of locos having undergone wheel turning more than once in a year should be prepared. Look out for repeat location in same loco or of same location in different locos, An analysis based on wheel location and service type wise loco utilization should be prepared. Wheel gauging and axle box clearances checking should be done in monthly schedule and records should be kept. Information regarding health & physical status of wheels or components is key to successful maintenance planning.

Factors to be checked:

·        Check wheels and their locations which have developed root wear. Check type of wear: If wear rate is slow it is called benign. It is characterized by low wear rates, minimal plastic deformation, formation of a surface film protecting against metal to metal contact & oxide wear debris. Severe wear is characterized by high wear rates, extensive plastic deformation, transfer of material to the harder counter face and flake like metallic wear debris.

·        What is trend of root wear increase? Identify locations having highest root wear / rapid growth rate.

·        What are longitudinal and lateral clearances at those locations? Are there excessive lateral clearances between axle box & bogie pedestal. Lesser longitudinal clearance between axle boxes bogie pedestal should be attended.

·        Check condition of bogie and axle box liners, Wrong setting of axle box wear liners or bogie frame pedestal lateral wear lines

·        Check wheel profile, wheel dia variation in same axle, conicity and surface finish of wheel

·        Bogie frame rotation, condition of centre pivots, side bearer,squareness/trammeling of bogie. Mis-alignment of bogie frame. Parellelness of bogie to be checked diagonal division variation should not be more than 3mm.

·        Biased wear on all wheels. If the root wear on one side of all six wheels is high compared to other wheels in same axle. The direction of loco is apparently not getting changed and loco is running in curved section

·        Wheel load distribution,  spring pairing and use of spring of equal loaded height in same loco,check equalizing/compensating beam movement and condition of rollers

·        Centre buffer coupler angularity

All unusul values should feature in exception list of undercarriage section.

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Attention during maintenance:

1.  Before doing wheel turning, axle box clearances should be checked. Lateral & longitudinal clearances of axle boxes should be kept within prescribed limit. Lateral clearances should be brought to shop issue size (minimum value) for locations exhibiting high flange wear.

2.  Locos provided with manganese steel liners at pedestals should be checked in M₄ & above schedule to ascertain their intactness & wear. Liner gap must be uniform. Axle box liners must be renewed during M₂₄ Schedule and above.

3.  If wheel profile has worn then wheel profile should be restored with minimum cut. Surface finish of wheel should be ensured during turning.

4.  Trueness of template used in wheel turning should be periodically                    cross-checked (every 6 months). Wear adapted wheel profile to be used.

5.  Wear of all pins and bushes must be kept within 1mm. Brake riggings should be renewed during M₂₄ schedule. Worn components should be replaced out of course whenever any play is observed.

6.  Alignment of brake hanger bracket on bogie frame should be checked during M₂ schedule for ALCO and in 90 days schedule for HHP locos.

7.  Uniform load distribution: Condition of roller at the end of the equalizing beams. Should be checked for wear, flatness and free movement. Rollers should be renewed during M24 and above schedules. Load tested springs of same group should be used. Side bearer pad gap must be same both sides.

8.  Condition of centre pivot and side bearer should be checked for sign of wear and free rotation. Regular adding of lubricant in the side bearer pans for free rotation of bogie should be done.

Implementation of above instructions during POH pre-commissioning and major/medium schedules will significantly improve wheel life and reduce the need of frequent wheel turning.  A seminar has been planned in last week of November at HQ on maintenance planning of bogie for arresting root wear.