Traction Rolling Stock Operation PDF

___________________________________________________________________________ 5. ADHESION It can be described in following ways :- 1. Adhesion is the grip or force of attachment, produced by friction between the wheels and rails. Adhesion is required to keep the...

___________________________________________________________________________ 5. ADHESION It can be described in following ways :- 1. Adhesion is the grip or force of attachment, produced by friction between the wheels and rails. Adhesion is required to keep the wheels from slipping. It depends on various factors and applies a limit on useful TE for a given axle-load. µ = T max / W Fig. 1 Coefficient of Adhesion Coefficient of Adhesion (µ) is the ration of “maximum value of Tractive Effort (Tmax ) which can be transmitted to the wheel” divided by the “effective value of load (W) on the driving axle” T max α W or T max µ W  max T µ -------------- w Max. Value of for steel on steel =0.44 = 44% in most ideal case ,.Therefore, if T max is to be increased, Weight on driving-wheel has to be increased, but track has the limitation of Max. Axle-load, therefore number of ales has to be increased, but for curvature. Traction Rolling Stock : OPERATION. 130 ___________________________________________________________________________ The Wheel Slip Phenomenon Bad effects of wheel Slip Damaged Gears - Damaged gear profiles lead to other modes of oscillations. Damaged bearings Cracks in bogie frames, supports and fixtures. Excessive wheel wear and rail-burns. Fig - Rail burn Traction Rolling Stock : OPERATION. 131 ___________________________________________________________________________ Fig. Rail burn Damaged Wheel Traction Rolling Stock : OPERATION. 132 ___________________________________________________________________________ Damaged Rail Head Damaged Rail Head Traction Rolling Stock : OPERATION. 133 ___________________________________________________________________________ FACTORS AFFECTING ADHESION 1. Effect of speed on adhesion :- As friction is maximum at start and then reduces with speed, similarly adhesion is maximum at start and then reduces with speed. If µas = Coefficient of adhesion at start & µar = its value at speed V, then as per SNCF :- 8+0.1 V µas = µas ------------------------------------- 8+0.2 V and as per Curtius & Kniffler :- 7.5 µar = -------------------------- + 0.16 V + 44 2. Rail condition and weather condition :- Wet rails reduce adhesion. Oily rails drastically reduce adhesion. A thin film of dust etc. gets stuck to wheel-rim and reduces adhesion-value of steel on steel. Dry leaves and coal dust also reduces adhesion. Moderate to heavy rain is better than drizzle for adhesion. Sanding helps, but the sand should be fine, dry and should fall on rail-head. Unevenness of rail-wheel contact, due to worn-out rail or wheel, loose track packing, warp in wheel-rim, difference in wheel-dia, irregular wheel tread profile, variations in track-levels, less contact area between rail and wheel at points and crossings, and curves causes the reduction in adhesion. Traction Rolling Stock : OPERATION. 134 ___________________________________________________________________________ Reduction of Adhesion on curves The angle subtended between the wheel flange and the gauge face of the rail is called “angle of attack”. Increase in this angle by 1 deg. As on curves, reduces the adhesion by half. Calculation of weight transfer Traction Rolling Stock : OPERATION. 135 ___________________________________________________________________________ M = Mass of locomotive at the centre of gravity. T = Tractive effort exerted by the motor at each driving axle. L = Bogie centre distance. I = Bogie wheel centre distance. H = Height of drawbar coupling above rail level. H = Height at which TE is exerted by the bogie on the locomotive body. Final weight distribution due to weight transfer between bogies and between axles of the bogie is as follows:- The weight transfer effect is reduced with the increase in the bogie wheel centre distance. Due to safety considerations of negotiating curves, points and crossings, wheel centre distance of bogie, can be adjusted only to a limited extent. In the bogie employing nose suspended motors, wheel centre distance is fixed by the diameter of driving wheels and traction motor dimensions. While reducing the value of ‘h’ will have desirable effect on the weight transfer between the two axle of the bogie, it will on the other hand increase the weight transfer effect between the bogies. Weight transfer between the axles of the bogie of conventional design is og the order of 15 to 20% of the adhesive weight of Traction Rolling Stock : OPERATION. 136 ___________________________________________________________________________ locomotive and weight transfer effect between the bogies is only 1 to 3%. Thus the overall effect due to the reduction in the value of ‘h’ is to decrease the weight transfer considerably. Methods of Reducing The Weight Transfer The weight transfer in the case of trailing axle of leading bogie and leading axle of trailing bogie are just opposite to each other. Thus by effecting vertical coupling between bogies by resilient component vertical reactions due to weight transfer are made to cancel each other. By means of low traction bas the point of application of tractive effort by bogie on the locomotive body is virtually brought down i.e. the value of ‘h’ is reduced. This design feature is incorporated in the manufacture of bodies of WAM1, WAG and WAG4 locomotives of Indian Railways. 3. Mechanical Factors : 1. Effect of weight transfer : When the loco is standstill on level gradient, its weight is equally shared by all axles, but this condition is disturbed when the loco or train is in a condition of run or start/brake, due to turning moments. Traction Rolling Stock : OPERATION. 137 ___________________________________________________________________________ Sr. Component Important factors No. 1. Height or Drawbar & Centre – pivot. 2. Gap bet. Loading pts. On front and rear 1. Body Reaction bogie. 3. Secondary suspension. 1. Direction of TM Noses 2. Bogie Reaction 2. Height of centre pivot. 3. Primary suspension. 1. Diameter of wheel. 3. TM Nose Reaction 2. Gap bet. TM Nose and axle-centre. 3. Direction of Noses. 3.1 (a) Effect of Truck Draw Bar Pull :- This results in reduction of load on leading bogie, and corresponding increase in load for trailing bogie :- (b) Effect of Traction Motor Nose :- If the direction of nose is towards the direction of motion of loco, the nose presses upward on bogie and equal pressure acts downwards on axle bearing, increasing pressure on axle. If the nose points opposite the loco-motion, the load on the axle decreases. TM Nose Force = N = (1092 / 2 ). (6 / 800) = 4.1 t Traction Rolling Stock : OPERATION. 138 ___________________________________________________________________________ WEIGHT TRANSFER DUE TO TORQUE EXERTED BY TRACTION MOTOR If the direction of motion is from left to right and D = Diameter of driving wheel. d = diameter of the gear wheel. S = distance between the axle and the nose. T = Tractive effort at the rails. Traction Rolling Stock : OPERATION. 139 ___________________________________________________________________________ The force at the gear teeth = TD/d and its direction is downwards on the gear wheel and its reaction on the pinion of the motor is upwards. As a result of this, motor nose exerts an upward force F on the bogie truck. When the vehicle is moving in the direction towards which the nose is pinting, the motor nose presses upwards on the bogie truck and axle bearing presses downwards on the axle, thus increasing effective axle-load. These forces are reversed when the vehicle is moving in opposite direction to that in which the nose is pointing. In WAM-4 and WAG-5 , both Nose-Reaction and Truck-Reaction are subtractive from the weight on leading axle, hence there will be tendency of lifting or wheel slipping. Motion for WAG – 5 In high-adhesion bogies of WAG-7, Nose-Reaction is adding to the weight of leading axle, so better adhesion and less chances of slipping occurs. Traction Rolling Stock : OPERATION. 140 ___________________________________________________________________________ Motion for WAG – 7 COMPARISON BETWEEN WAG – 5 & WAG-7 Max. T.E. (without wheel-slip) = No. of axles x Min. Net load on axle For WAG-5, 6x14.41x0.37 = 32 t, and For WAG-7 – 6 x 19.16 x 0.37 = 42.5 t Traction Rolling Stock : OPERATION. 141 ___________________________________________________________________________ 3.2 Effect of vertical shocks :- The contact between rail and wheel gets detached, under the effect of instantaneous vertical shocks. Provision of better elastic suspension and damping arrangement in a bogie, reduces the chances and duration of such a loss of rail-wheel contact, thus giving better adhesion. Primary suspension of WAG-7, has sets of equalizers hung directly on end axle boxes, and supported on middle axle box though a link and compensating beam arrangement. This ensures equal distribution of vertical load on all 3 axles. WAG-5 has two different sets of equalizer beams, one each between either end-axles and middle axle. Secondary Suspension of WAG-7 has 4 side beares on each bogie, and share full vertical load leaving nil for centre pivot. In WAG-5, centre pivot takes 60% of vertical load, and 40% is shared by 2 side bearers. Suspension System of Locos CO-CO TIRMOUNT BOGIES WAM - 4 , WAG - 5 LOCOS HIGH ADHESION BOGIES WAG - 7 LOCOS BOGIES WITH BOLSTER WAP - 1, WAP - 4 BOGIES OF THREE PHASE LOCOS WAP - 5 WAG - 7 CO-CO TRIMOUNT BOGIES OF WAM - 4, WAG - 5 Side bearers - 40% load Centre pivot - 60 % load Snubbers - 4 per bogie Equalizing bear Motor axle hung nose suspended Only primary suspension Unequal Wheel Base Traction Rolling Stock : OPERATION. 142 ___________________________________________________________________________ HIGH ADHESION BOGIES OF WAG - 7 Side bearers - 4 Nos per bogie : Carries 100% load equally divided. Centre pivot : Carries only TE/BE and no vertical load. Equal wheel base : 3800 mm (1900+1900) (Unlike 1702+2108 for trimount bogie) Primary and Secondary suspension T.M.axle hung nose suspended Lateral Dampers between Body and Bogie Leading and trailing axle box bearings are provided with rubber thrust pad. Traction Rolling Stock : OPERATION. 143 ___________________________________________________________________________ FLEXI-COIL BOGIES OF WAP - 1 / 4 Primary and Secondary both suspension. Provision of bolster. Secondary suspension has got lateral friction dampers. Primary suspension with helical springs. Bolster is supported on springs provided on Bogie frame. Low traction - Bar Traction Rolling Stock : OPERATION. 144 ___________________________________________________________________________ BOGIES OF WAP 5 / WAG 9 Primary and Secondary suspension. Primary suspension - Springs and damper Secondary suspension - Springs and damper Lateral dampers Yaw dampers WAP - 5 : Motors are fully suspended. WAG - 9 : Motors are Axle hung nose suspended. 4. Electrical factors 4.1 Effect of performance characteristics of TMs : The steepness of the TE vs Speed characteristics of TM, decides the time taken for arresting the wheel-slip and better adhesion. Normally TE at any speed should be lower than the maximum adhesive limit, but if the maximum adhesive limit decreases due to factors like dew or oil on rails, the wheels start slipping, and speed increases, causing TE to fall. If this instantaneous fall in TE is too rapid, it may become lower than the new adhesive limit and slipping may be arrested, otherwise for a less steep curve the slipping will continue a longer. Traction Rolling Stock : OPERATION. 145 ___________________________________________________________________________ 4.2 Effect of TM combination in series or in parallel : Wheel-slipping of one axle causes the speed of that TM to increase, in turn increasing the back-emf, thus reducing the current. Now if TM groups are in series, the current-reduction in slipping TM will also cause current-reduction in other TM in series with it, so developing slipping in additional TM. Whereas TMs in parallel will not be affected by slipping of one TM. Hence 6 - P combination of TMs give better adhesion than 2 S - 3 P combination. 4.3 Medhod of traction control : Method of control of TM by rheostatic method as in DC locos, or by tap- changer method as in AC locos, causes sudden large variation in TE in discrete seps. Then, the average value of TE becomes much less than the maximum permitted by adhesive limit. Increased number of steps reduce the variation in TE and hence the average value of TE rises and becomes closer to maximum. Continuous step-less control, as provided in 3-phase locos achieve better adhesion. 4.4 Use of Switch ZQWC in Loco, by Driver : The torque developed by traction motor (TM) is proportional to the product of field flux and armature current. By use of ZQWC a part of the TM’s field is diverted through a shunting resistor. Therefore torque produced by motor and consequently TE at the corresponding wheels will be lower. In this way, fields of TMs on off-loaded axles are weakened, while those of TMs on overloaded axles are working to Traction Rolling Stock : OPERATION. 146 ___________________________________________________________________________ their full strength. Thereby total TE of locomotive is so distributed among axles that ratio of TE to weight is more or less equal for all axles. This provides relief to offloaded axles that would otherwise have this ration unduly strained - heightening the probability of wheel-slip. For same level of limiting adhesion - utilization - factor ( ) , higher TE can be obtained from the locomotive. Circuit for Weight Transfer Compensation Switch A spring-loaded switch named ‘ZQWC’ is provided on the Driver’s desk. The Driver is expected to use it by pressing it until the train starts rolling while starting the train on up gradients. This switch operates a relay ‘QWC’, which in turn operates the shunting contractors to achieve shunting of fields of desired TMs depending upon the direction of motion. It may be noted that the Driver is supposed to leave the switch, the moment locomotive has begun to roll. Therefore this circuit is relevant only before the moment in which back emf gets established. 4.5 Enginemanship : The Driver’s skill or enginemanship also affects the adhesion while in motion. Sudden increase in TE may result in a value higher than permitted by Kinematic Coefficient of Adhesion and may result in slipping and auto- regression, finally causing temporary reduction in TE. Negotiating a gradient with necessary attacking speed and timely use of sanders, helps in maintaining proper adhesion. Methods to improve Adhesion 1. Bogie Design 2. Selection of TM Characteristic 3. Arrangement of TMs in series or parallel 4. Field weakening of slipping TMs 5. Engineman-ship 6. Stepless Control 7. Sanding 8. Creep Control SLIP- SLIDE CONTROL OR CREEP - CONTROL Maximum tractive or braking effort is obtained if each powered wheel of the vehicle is rotting at such an angular velocity that its actual peripheral speed is slightly higher (motoring) or slightly lower (braking) than the true vehicle speed (i.e. the linear speed at which the vehicle is traveling, usually referred Traction Rolling Stock : OPERATION. 147 ___________________________________________________________________________ to as “ground speed” or “track speed”. The difference between wheel speed and track (or “ground”) speed is referred to as “slip speed” or Creep. This system and method maximizes the available rail adhesion between the rails of a track and the wheels of a rail vehicle so that the vehicle is better able to accelarate up to operating speed and to decelerate to a stop condition under poor rail conditions. Doppler radar based control : Measure Vehicle speed independently. Measure individual axle speed. Control slip by permitting wheel to slip at fixed rate above vehicle speed. Examples : WAG - 6 A, WDG - 4. Delta-N Control Estimate reference speed. Obtain TE feedback. Permit slip till TE is maximum. Examples : WAG- 6 B and C, WAG - 9 , WAP- 5 SLIP- SLIDE CONTROL OR CREEP - CONTROL There is a relatively low limit value of slip speed at which peak tractive or braking effort is realized. This value, commonly known as maximum “creep speed,” is a variable that depends on track speed and rail conditions. So long as the maximum creep speed is not exceeded, slip speed is normal and the vehicle will operate in a stable microslip or creep mode. Traction Rolling Stock : OPERATION. 148 ___________________________________________________________________________ If wheel-to-rail adhesion tends to be reduced or lost, some or all of the vehicle wheels may slip excessively, i.e. the actual slip speed may be greater than the maximum creep speed. Such a wheel slip condition, which is characterized in the motoring mode by one or more spinning axle-wheel sets and in th3e braking mode by one or more sliding or skidding axle-wheel sets, is continuously monitored, detected and corrected immediately. STEPS IN SLIP / SLIDE CONTROL 1. Measuring actual vehicle velocity and computing there from values of wheel rotational velocity and wheel acceleration for the actual vehicle velocity ; 2. Measuring actual wheel velocity and deriving there from actual wheel acceleration; 3. Determining if actual wheel velocity varies from the computed wheel velocity by more than a selected first minimum value and, if so , generating a wheel slip/slide signal. 4. Computing, in response to the slip/slide signal, a difference between actual wheel acceleration and computed wheel acceleration; and 5. Summing a value representative of the computed difference with the torque request signal so as to adjust motor torque in a manner to correct the wheel slip / slide condition. 6. Determining if the actual wheel velocity varies from computed wheel velocity by more than a second minimum value greater than the first minimum value and, if so, substantially reducing the torque request signal until the variation between actual and computed wheel velocity is less than the 2nd min. value. 7. Inhibiting modification of the torque request signal until vehicle velocity exceeds a minimum threshold value. 8. The first minimum value includes a slip value and a slide value and the method of determining includes comparing actual wheel velocity to each of the slip and slide values for generating respective wheel slip and wheel slide signals. 9. Determining if the vehicle is in a propulsion or in a braking mode and enabling a corresponding one of the slip and slide signals. ------------------ Traction Rolling Stock : OPERATION. 149