A Technical Note in Continuation of Reasoned Document of EOI of AAR ‘H’ Type Tight Lock Couplers having Balanced Type Draft Gear

Issued August 2011

Prepared by:

Anand Swaroop Director, Carriage Directorate,

RDSO Disclaimer: This technical note has been prepared by Research Designs and Standards Organisation, a R&D wing of Indian Railways. The views expressed in this technical note are subject to modifications, in the light of fresh data, from time to time. Further, they do not necessarily represent the views of Ministry of Railways (Railway Board), Government of India. This report is property of RDSO and is meant essentially for official use. It may not be loaned, reproduced in part or full, or quoted as an authority without prior written permission of Director General, RDSO, Lucknow.

CONTENTS

Para Page

1.0 Background 1

2.0 Comparison of key parameters of various types of draft gear in service on IR with balanced type draft gear in new specification

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2.1 Type of draft gear 2

2.1.1 Single pack draft gear 2

2.1.2 Floating plate or twin pack draft gear 3

2.1.3 Balanced Type draft gear 3

2.1.4 Alternative Design of balanced type of draft gear 5

2.2 Energy storage capacity of draft gear in revised specification 5

2.3 Linking arrangement between coupler and draft gear 7

3.0 Summary and conclusion 9

Annexure –A 10

Annexure –B 11

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1.0 Background

The screw coupling with side buffer was the only coupling arrangement in mainline BG passenger trains on Indian Railways (IR) till year 2000. AAR ‘H’ type Centre Buffer Coupler (CBC) was first time used in 24 imported Alstom/ LHB coaches. This type of coupler was chosen because of its compatibility with the couplers of locomotives. CBC in Alstom/ LHB coaches were supplied by Schaku with a Ringfeder draft gear. These draft gears had total energy storage capacity of 30 kJ. After the introduction of these coaches in commercial service, problem of train partings due to vertical slippage with the AAR E type couplers fitted on locomotives, opening of couplers due to defective design of anti-creep and locking mechanism, rapid wear of the coupler shank hole and the pin at the coupler and draft gear connection appeared because of defective design of CBC.

A new specification no. CK009 was formulated by RDSO in 2000 to take care

of problems faced in the couplers fitted in Alstom/ LHB coaches. A restrictor was also provided on the couplers of the end coaches to prevent uncoupling due to vertical slippage with the loco couplers. The total energy storage capacity of these draft gears was 45 + 5%

-5% kJ which was later revised to 45 kJ. The couplers to this new specification have been mostly supplied to IR by M/s Faiveley, M/s ASF-Keystone and M/s Escorts. Draft gears of the couplers supplied by these firms have mostly single pack designs. The approximate population of different type of draft gears by mid of year 2011 is as below:- Type of draft gear

Population (Coach sets)

Service Since

Single pack 2560 Year 2001 Twin pack 325 Year 2004 Floating Plate 30 February 2010 Balanced type 22 February 2011

60 nos. of floating plate type draft gears supplied by M/s Faiveley are mostly in service in ALD-NDLS Durranto Express. 60 nos. of balanced type draft gears supplied by M/s Dellner are mostly in service in Secunderabad- Mumbai Durranto Express. Longitudinal jerks in mainline coaches equipped with CBC have been a major concern since introduction of CBC. Several measures were taken in the past to reduce the longitudinal jerks but they resulted in minor improvements only.

On the basis of a systematic study of design of CBC, a new specification of CBC has been made. In the new specification, balanced type draft gear has been specified. CBC conforming to the new specification is expected to address the problem of longitudinal jerks arising out of CBC.

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2.0 Comparison of key parameters of various types of draft gear in service on Indian Railways with balanced type draft gear in new specification

The specification of CBC CK-009 (Rev.2) does not specify the type of draft gear. Suppliers are free to supply any type of draft gear such as single pack, twin pack, floating plate or any other design as long as it meets the specified requirements of end force and energy storage capacity.

In the new specification, the ambiguity in type of draft gear has been removed. The type of draft gear has been specified as ‘Balanced ‘. The word ‘Balanced’ is a common word but it has been defined in context of draft gear in RDSO sketch no. CG-K8207 alt.1. In the following sections, the key parameters of the specification CK-009 (Rev.2), which have been changed in the new specification, have been explained along with the reasons.

2.1 Type of Draft Gear 2.1.1 Single Pack Draft Gear In this type of draft gear, same set of draft pads is used in buff as well as in

draw mode. The problem of longitudinal jerks because of using this type of draft gear arises because in dynamic condition, the front follower leaves the front stopper and the rear followers leaves the rear stopper during buff and draw modes respectively and hits them on load reversal. Also, preload in the draft gear adversely affects the longitudinal dynamics of a train. In passenger coach, preload should be effectively zero. However, in single pack draft gear reducing preload beyond a certain value may be counterproductive as preload may lose significantly over a period of time in service and problem of follower hitting the stopper may further deepen.

part from the problem of slack in dynamic condition, the following diagram of conceptual draft gear characteristics shows that there is abrupt change in force in load reversal adding to the problem of jerks.

Figure-1: Travel Characteristics of Single Pack Draft Gear

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2.1.2 Floating Plate or Twin Pack Draft Gear Floating plate or twin pack draft gear has similar cushioning arrangement. In

floating plate or twin pack draft gear, the problem of rear follower plate striking the rear stopper has been addressed by dividing the draft pads into two parts by a floating plate and using this floating plate for the purpose of transmitting force in draw mode. In draw mode, only draft pads between floating plates and front follower is compressed whereas in buff mode, all the draft pads are used to take buff load. It has zero preload in draw mode.

However, in buff mode, this type of draft gear behaves in a way similar to

single pack draft gear. In dynamic condition, the front follower leaves the front stopper and hits it on load reversal. Also, preload in the buff mode adversely affects the longitudinal dynamics of a train. Apart from the problem of slack in dynamic condition, the following diagram of conceptual draft gear characteristics shows that there is abrupt change in force in load reversal though the extent of change is moderate compared to that of single pack draft gear.

Figure-2: Travel Characteristics of Floating Plate Draft Gear

These attributes have an effect on sustainability of performance related with longitudinal train dynamics of a train. IR has experience of using twin pack draft gear since year 2004 and its performance with respect to longitudinal behaviour has not been satisfactory.

2.1.3 Balanced Type Draft Gear

Balanced type draft gear overcomes all the problems mentioned in the above sections. The draft gear is fixed between the front and rear stopper and no relative movement between the draft gear frames and these stoppers is

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possible. There is no situation of front follower leaving the front stopper and rear follower leaving the rear stopper and hitting them on load reversal. Apart from this, the following diagram of conceptual draft gear characteristics shows that there is no abrupt change in force on load reversal. Force travel curves are regular and they are not vertically separated at neutral position. It facilitates smooth load reversal.

Figure-3: Travel Characteristics of Balanced Type Draft Gear

Because of above advantage of balanced type of gear, it is expected to perform better from longitudinal train dynamics point of view and the performance is sustained. This has been chosen for the new specification.

Following are the subjective assessment of relative performance of various types of draft gear Performance Criteria# Single Pack

Draft GearTwin Pack Draft Gear

Balanced Draft Gear

Energy storage Capacity in Draw mode ***** *** *** Energy storage Capacity in Buff Mode ***** ***** *** Longitudinal Train Dynamics (Jerk Free) * ** *****

# - Higher number of * means better attributes.

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2.1.4 Alternative Design of Balanced Type of Draft Gear

Draft gear having following arrangement can provide an alternate way of having balanced type arrangement.

Figure-4: Alternative Design of Balanced Type Draft Gear

However, this has not been considered in the new specification because of following reasons:- a) The front and rear followers are not rigidly fixed with front and rear

stoppers respectively. b) Value of pre-compression cannot be accurately controlled. Also, it

depends on distance between front stopper and rear stopper. c) It needs heavy draft gear frame d) Not suitable for single support arrangement as envisaged in preamble of

new specification for reducing the cost. 2.2 Energy Storage Capacity of Draft Gear in the Revised Specification

The minimum energy storage capacity of draft gear in new specification has been reduced from 45 kJ mentioned in specification CK-009 (Rev.2) to 35 kJ. The 45 kJ energy storage capacity is more than the required. Following is the basis of deciding minimum energy storage capacity of 35 kJ:- a) For deciding the minimum energy storage capacity of a draft gear, shunting speed of locomotive is the prime consideration. Length of a train or number of coaches in a train does not play any role as in train operation only a fraction of energy storage capacity is utilized. b) In Technical Bulletin No. MP.IB.LD.01.17.10 (Rev.00) dated 07.04.2010, the recommended shunting speed is 2-3 Kmph. Assuming weight of locomotive is 112.8 t and weight of a single coach is 64 t, the energy storage requirement of a draft gear when equally shared between locomotive draft gear and coach draft gear for a speed of 3 kmph is 7 kJ (Energy storage

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(E) = Shunting speed 2 / 1.27). So the energy storage of 35 kJ in the new specification is 400% more than the minimum required value. The shunting speed equivalent to 35 kJ comes out to be 6.67 kmph which is more than the double of the prescribed shunting speed.

c) A coach has inherent energy storage capacity because of its elasticity. It is designed to take a minimum 2000 kN buff load without any permanent deformation. Actual coach strength is much higher than this value. FEM analysis of ICF as well as LHB coach indicates that the coach has longitudinal stiffness (buff to buff) of 154 kN/mm. This indicates that the coach can store elastic energy equal to (Energy Stored = 1/2XForce2/Stiffness) 13 kJ when a buff load of 2000 kN is applied at one end and buff stopper at the other end is restrained. Assuming only half of this energy i.e. 6.5 kJ remains available when the coach is impacted from one end and it is free from another end, the equivalent shunting speed is 2.9 kmph. The calculation suggests that up to a shunting speed of 2.9 kmph coach will not undergo any permanent deformation even if energy storage capacity of draft gear is zero. d) IR has long experience of coupling system in mainline coaches with screw coupling and side buffer. Side buffer has equivalent energy of 20 kJ per coach end which was increased to 25 kJ in year 2004. The instruction on shunting speed has not changed over a period of time. e) In 24 LHB/ Alstom coaches imported under TOT in year 2000, the draft gear capacity had 30 kJ (minimum) energy storage capacity. f) Wagon Directorate is importing a state-of-art draft gear as per AAR specification AARM901E from USA for its premier wagons like BOXN, BOXNHL, BCNHL etc. The wagon has maximum gross weight of 100 t. The energy storage capacity of the draft gear is 48.8 kJ. This is equivalent to shunting speed of 6.9 kmph. This shunting speed is applicable to a freight stock where impact conditions are much more severe because of various reasons. It can be seen from this that 45 kJ energy storage capacity in the old specification had higher shunting speed in consideration than even that of freight wagon. g) High energy storage capacity necessitates use of all the available cushioning pads in draft gear in case draft gear housing space restricts more number of pads. Floating plate draft gear, twin pack draft gear and single pack draft gear use all the pads in buff mode. However, these draft gears are not considered suitable from longitudinal train dynamics point of view.

Balanced type draft gear uses a separate set of pads in draft and buff modes and thus requires total more capacity of pads for absorbing same amount of energy in buff mode. However, it is seen that with high quality of elastomeric rubber pad or innovative design of draft gear where articulation is inside the draft gear, it is possible to achieve higher energy storage capacity though it is not required to have more than 35 kJ. It will unnecessarily increase the maintenance cost because of replacement of pads and cost of

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coupler itself. Also, draft gear characteristics may not be optimized for the best longitudinal train dynamics.

2.3 Linking Arrangement between Coupler and Draft Gear

Function of linkage arrangement is to enable coupler head to swing horizontally as well as vertically as draft gear is fixed to underframe of a coach. In RDSO specification CK-009 (Rev.2), the linkage arrangement is not specified. It has resulted in supply of CBC having stabilizing link which is not considered good for longitudinal train dynamics. Stabilizing link supplied by M/s Faiveley is itself a source of jerk. The small stabilizing link spring has lower preload than draft gear pre load. The arrangement provides 10-12 mm slack in dynamic condition at the time of load reversal from buff to draw mode. The stabilizing link acts like a mini single pack draft gear and brings all the evils of single pack draft gear with it. This small spring has the potential to set permanently over a period of time and needs replacement in maintenance and adds to the cost. It becomes the first element to lose its preload. Stabilizing link in case used with the balanced type of draft gear creates a reverse situation in which draft gear has effectively zero preload but stabilizing link has preload of 25 kN. The new specification specifies linking arrangement between coupler and draft gear to overcome this problem. Spherolastic bearing prescribed in the specification meets the functional requirement of articulation. Apart from its role of providing articulation to coupler head, it helps in blocking transmission of vibration and noise to coach body. Socket type fixed joint between coupler shank and draft gear is another type of linking arrangement prescribed in the specification. In this option, the articulation happens inside draft gear. This arrangement further simplifies the whole design. IR has experience of using both the types of linking arrangements and they are satisfactory.

Figure-5: Spherolastic Bearing

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Various doubts have been raised on using articulation inside draft gear. Analysis of LHB coach on 10 degree curve indicates that geometrical requirement of coupler head movement because of curvature of track is 3.9 degree horizontal swing. The estimated bending force because of this horizontal swing is insignificant as compared to bending strength of the shank prescribed in the specification.

Figure-6: Horizontal Swing of Coupler in Coaches at 100 Curve

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In many coupling system design, some stiffness and damping in horizontal swing is desirable to prevent yaw of a coach during the taking of curve, transitions and switch points.

Figure-7: Balanced Type Draft Gear while Negotiating Curve

3.0 Summary and Conclusion

Longitudinal jerks in mainline coaches equipped with CBC have been a major concern since introduction of CBC. Several measures have been taken in the past to reduce the longitudinal jerks but they resulted only minor improvements.

In section 2.1 above, technical analysis with respect to longitudinal train dynamics of various types of draft gear presently in service on IR has been done. It is seen that balanced type draft gear provides the best solution for passenger coaching stock. The minimum energy storage capacity of draft gear in new specification has been reduced from 45 kJ mentioned in specification CK-009 (Rev.2) to 35 kJ. Section 2.2 deals with minimum energy storage capacity of draft gear in detail. It explains that 35 kJ is adequate for passenger coaching stock. Linking arrangement between coupler and draft gear is another key change that has been made in the new specification. In RDSO specification, CK-009 (Rev.2) the linkage arrangement is not specified. It has resulted in supply of CBC having stabilizing link which is not considered good for longitudinal train dynamics. Section 2.3 deals with this issue in detail. The new specification specifies use of spherolastic bearing or socket type joint for linking coupler with draft gear to overcome the problem of longitudinal jerks. On the basis of a systematic study of design of CBC, a new specification of CBC has been made.CBC conforming to the new specification is expected to address the problem of longitudinal jerks arising out of CBC.

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ANNEXURE-A

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ANNEXURE-B

Calculation of Impact Energy

Weight of a Locomotive = WL in ton Weight of a Vehicle = Wv in ton Initial Velocity of The Locomotive = VL in kmph Initial Velocity of The Vehicle = Zero Initial Kinetic Energy = 38.5 x 10-3x WL x VL

2 kJ Assuming both the vehicle moves at a common speed of VC after impact Residual Kinetic Energy = 38.5 x 10-3xVL

2x WL 2/ ( WL+ Wv) kJ

Kinetic Energy Lost = 38.5 x 10-3xWL xWv x VL

2/ ( WL+ Wv) kJ The energy storage requirement of a draft gear when equally shared between locomotive draft gear and coach draft gear = Kinetic Energy Lost / 2 = 19.25 x 10-3xWL xWv x VL

2/ ( WL+ Wv) kJ

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