Indian Railway Context – Sanding to improve adhesion

In Indian Railway context, sanding is one of the important design and functional requirement to improve adhesion between rail and wheel during traction. The requirement has gone so deep in the background of policy makers that it is difficult to believe a traction equipment without sanding gear. Any case of an investigation into wheel slipping/stalling  starts with the question of working of sanders and finishes there itself. The working of sanders is one of the item of the drive before the onset of monsoon and winter dew.  Unfortunately, to what extent it has benefited, has never been  studied or analysed effectively to conclude on the following aspects:

1. impact of sanding in improving adhesion;
2. specification of sand to be used;
3. where and when to use sanding;
4. quantity of sand in one fill and how long it will last;
5. number of sanders sufficient, i.e. 2, 4, 8 or 12 for the leading; and
6. relevance with different class of locomotives.

When the first series of WAG1 locomotive imported from France, the manufacturer studied the types of sand available nearby and zeroed down to Damador River sand near Asansol. Later on, with electric traction extended to all places, the relevance of specified sand lost its relevance. Are we sure the sand used is really serving the purpose?

System of slip-slide control of Locomotive

Tap Changer Locomotive: The system of slip (not slide) control was provided from the earliest series of WAG1 class of the locomotive. During wheel slipping, the difference between two traction motor current is detected, and if more than the permissible limit, the wheel slip lamp glows resulting in auto-regression of tap changer and auto-sanding. The auto-regression of tap changer ranges from 4 to 10 notches and during the process, sometimes the wheel slips by 8 to 10 rotation causing damage to the rail.

Uncontrolled wheel slipping results in heavy damage to rail requiring early replacement. A video below shows the grinding action of rail due to wheel slipping.

GTO/IGBT Locomotive: The system of slip and slide control is provided in which the wheel slipping or sliding is detected within half a rotation and traction inches down to match perfectly with the adhesion available. This is accompanied with auto-sanding also. The damage to rail table is negligible with minimum amount of sand consumption. The problem only arises when the train is hauled on a gradient and the adjusted traction not sufficient to negotiate the gradient and in such case, the train comes to a stop and called stalling.

Initially, GTO locomotive has one block of three traction motor fed by one set of converter-inverter and traction  controlled for the all the traction motor of the block. But in IGBT based locomotive, each of the traction motor is fed by separate inverter, and therefore, slipping on one wheel results in drop of traction of that wheel only. This reduces the probability of continuous wheel slipping and stalling.

It is important to observe that the problem of wheel slipping is associated mostly with locomotive hauled trains. With distributed power, such as metro or train set, it is the drop in braking effort, the reduction of which may result in over shooting of  a signal and a safety issue. For this reason only, the braking effort calculation is done based on 25% adhesion so that chances of overshooting a signal is avoided.

Quality of Sand

Quality of sand plays an important role in improving the adhesion, not from 35% to 40% but 05-15% to 25% so that train can negotiate the gradient at a lesser speed. No studies have been done by Indian Railways to establish the right quality of sand and depending only on the works done by developed railway systems.  Railway Group Standard GMRT-2461  June 2016 details the specification applicable for braking and traction. and as follows

It is important to note that the size range for braking is courser i.e. 90% between 0.71-2.8 mm, whereas for traction, finer size is acceptable i.e. 70% between 0.15-1.18 mm. This is attributed to the following reasoning, based on the operational experience of British Railways and stated on the said specification.

Braking: The grain size tends to be larger than the rail contaminants layer, causing the grain penetrating the contaminant layer providing a mechanical link between wheels and rails.

Traction: The fine grain tends to form a paste between the wheel and rail, which is more effective at allowing the transmission of tractive effort between the rail and wheels.

Sand used for braking can also be used for traction, but not vice versa.

It is the experimental studies of working with different sizes of sand which will confirm what is stated in the said document.

Indian Railway Context – Actual sanding practice

Impact of sanding in improving adhesion: There is no systematic study available except that of few railway enthusiasts. It is my personal experience, that when train stalls and wheel slipping not permitting the train to start, the Loco pilots are isolating wheel slip detection mechanism. This prevents auto-regression and even when one or two wheels slip and other wheels continue to provide rolling, the train somehow clears the hurdle. The Assistant Loco pilot keep on putting small ballast pieces on the rail, which provides adhesion  and that provides some help. Does it mean that when small pebbles of ballast providing help, means a study of the composition of ballast material and working on the specification of sand.

Specification of sand to be used: There is a large variety of sand used and can be best be known if a sample is collected from different corners of the Railway network. Indian Railway is having a very large sand filling network establishing its extensive use. It is used on the Locomotive as a cheap item, extensive use and gets exhausted in just 15 minutes of use. Yes, it provides confidence to the Loco pilot that I am doing the sanding. If for some reasons, the train starts, he has the satisfaction that it is because of sanding.

From the video below of the sanding in process, it is clear that the sand used contain heavy dust, particle size very low and may not be effective. It is important to have the sand of the right size and to come in service only when the wheel slips automatically. The manual sanding shall be completely stopped. The sand proposed to be used will certainly be costly and be around Rs. 15-20 /Kg.

Where and when to use sanding: Manual sanding shall be prohibited. It should be used only through automatic sanding mechanism that too when the applied tractive effort just balances 35% of the adhesion. With right quality of sand used, which will certainly be costly, automatic sanding is the only preferred choice. The quality of sand for braking should look similar to the picture below

How many sanders are required: Sanding is required for the wheel which is slipping or tendency of slipping. Detection of the wheel slip is based on comparison of traction motor current in tap changer locomotive and anugular velocity of wheel in IGBT locomotive, therefore, it is not possible to identify the wheel which is actually slipping. In case, the second or third wheel is slipping, and sand is applied on the leading wheel, it will not serve any purpose, because the sand is already reduced to fine size. It is difficult to provide and maintain sander for each wheel. In view of this, IR decided to go for bogie wise sander i.e. 4 for the leading but effectiveness is still questionable.

With the introduction of precise slip-slide control introduced along with IGBT based individual control motors, the problem of wheel slipping and stalling of trains shall be negligible. Correct quality of sand  with controlled usage shall be only used while negotiating gradient and train experience loss of adhesion. 

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