Wheel slipping occurs when tractive effort exceeds adhesive weight whereas sliding occurs when braking effort exceeds adhesive weight. In both the situations, it is the adhesive weight playing the most important role.
Adhesive weight is defined as the force that can be exerted by a wheel without slipping or sliding.
Adhesive weight = Coefficient of Adhesion x Weight
Weight is limited by track structure and permissible axle loading which in the present context of Indian Railway is 20.3T. There is a plan to go for 25T axle load in heavy haul sections. Maximum possible adhesion with advanced electronics possible had been 0.40; therefore maximum tractive effort possible is 8.12T/axle at start.
When Tractive effort is more than adhesive weight, difference in power accelerate the wheel which results into grinding action on the rail. In the similar manner, when braking effort exceeds the adhesive weight, extra braking force prevents its rotation but with continuation of linear motion which results rubbing of wheel at one location on the circumference and called development of wheel flat. Both these conditions create unsafe situation.
Effect of weather on Adhesion
Weather and environmental conditions plays a vital role in reducing adhesion. There are always some impurities on rail table which when interact with moisture results drop in adhesion to as low as 0.10. The other conditions that has similar effect are
- Saline water dipping over the rail table from wagons during rainy season.
- Dry leaves are light and sucked by the draft created by the train and get crushed on the rail table. These dry leaves will get removed by the following train but if there is rain, it will get stick on the rail table resulting into drop in adhesion to very low value. Show on the picture crushing of dry as well as green leaves due to heavy storm and trains unable to negotiate gradient section of Manoharpur-Posoita Section of Chakradharpur Division of South Eastern Railway.
These conditions are seasonal during dry leave season (March-April), rainy season (July-Sept) and dew (Dec-Jan).
The condition keeps on varying from time to time in the same weather condition and is never constant. The churning effect of rolling of wheel and micro slip results in cleaning the deteriorated condition of the wheel continuously.
Experiencing wheel slip is more on circumstantial conditions of the prevailing conditions of adhesion and demand for tractive effort. Rail condition improves if a train requiring less adhesion is allowed to run in the section thus improving adhesion to benefit subsequent trains.
Statistical analysis plays an important role in identifying major causes for wheel slip based on section, weather condition at the time of wheel slip, load, type of stock, skill of the loco pilot etc.
Protection towards slipping and sliding
Protections towards wheel slip and slid lies with its early detection and reduction of tractive effort and braking effort respectively within least possible time.
The issue of protection towards wheel slip lies with traction wheels only i.e. locomotive. Different methods are used for detection of wheel slip in different class of locomotive depending on the technology that was existing at that time. With protection system invariable effective, damages on account of wheel slip is very less but may result in stalling of train if the need for tractive effort continuous to more than the adhesive weight.
Protection in Tap Changer Locomotive (WAG5,WAG7)
A slipping wheel will generate higher back emf thus less traction current and wheel slipping can be detected by comparing traction current.
All Traction Motor works in parallel in the present configuration of Locomotives. Relay QD1 and 2 are provided which senses the TM current of 1-3 & 4-6 in WAG5A&B locos respectively and energizes when it goes more than 180A, sensing a condition of wheel slip. Three actions takes place simultaneously namely
1. Glowing of LSP
3. Auto-regression of tap changer reducing TE of all motors till such time current difference drops to 150 A. This auto-regression may be 1 to 8 notches
Loco Pilot notches up slowly in readiness to regress notches manually and resort to manual regression, if there is tendency of wheel slip.
There is no protection towards sliding and there are many instances of wheel flat on locomotive. Preventive methods followed are
- Isolation of loco brake when ever train brakes are applied by pressing a pedal
- Isolation of loco air brake whenever dynamic brakes are used.
- Reducing braking effort when ever locomotive brakes are applied in synchronism to train brake.
Protection in WAG9 and WAG6 class of locomotive
WAG6 class of locomotive are Thyrister based with DC separately excited series motor and WAG9 class of locomotive is equipped with GTO/IGBT with regeneration. Detection, reduction and restoration of TE goes simultaneously thus making best utilisation of adhesion available. WAG9 class of locomotive is equipped with slide control also during regenerative braking.
There is no protection for wheel slide on coaching and wagon stock. Therefore there are more chances of wheel skidding for any malfunctioning of braking system. In recently introduced LHB coaching stock, slide control is provided and braking effort of the effected wheel is reduced on detection of wheel slide.
Coefficient of Adhesion vs speed
Coefficient of adhesion varies with speed as per the empirical formula given below:
It may be seen that the coefficient of friction reduces to 90%, 78% and 69% at speed of 10, 30 and 100 kmph respectively. Coaching trains running at 100 kmph experiences wheel slip even with slight dew on track.
Driving Techniques to prevent wheel slipping and stalling of trains
- Be alert while working on up grades
- Attack the gradient using full acceleration reserve (full use of short term thermal rating of the traction motor) to attain maximum permissible/possible speed before the start of the gradient. A train running with a speed V possesses a velocity head of app.
Factor 0.78 accounts for the head loss due to natural train resistance. Velocity head gives the idea the train will climb the vertical distance if allowed to roll freely.
A train running at 60 kmph or 16.66 m/sec possesses velocity head of
meaning thereby that the train will climb a distance of 1.1km if allowed to roll on 1:100 gradient. The important of velocity head is that it increases in proportion to the square of the velocity as given in the table below
|Speed kmph||Velocity Head in m||Linear distance on 1/200 grade||Liner Distance on 1/150 grade|
- Speed restrictions of 60kmph was imposed on loaded BOXN train while permitting CC+8 tonne loading in order to avoid any damaging effect on rolling or track.
- Total number of stretches on the entire Indian Railway territory may not be many and Railways shall consider increasing the maximum permissible speed on a stretch of 4 km before and few km on the grade.
- This arrangement will help in negotiating the gradient with application of less tractive effort which in all probability be within adhesive weight during inclement weather also.