Choosing your ROLLER
By Alex Vickers
If, like my club, you are strapped for cash, then having the luxury of choosing a roller might be a little unlikely. But, assuming you do have a choice, what are some of the key issues in respect to the effect of the roller on your pitch? Here are a few guidelines that might be of assistance:
Weight:
The terms heavy and light roller are a little meaningless to be honest. Weight of the roller is easily measured but that tells you very little about the compressive effect of the roller as it passes over the soil. The crucial thing is the contact pressure of that roller - essentially, this is the force applied by the roller (its weight) divided by the contact area (its footprint). You might have a heavy roller but, if that weight is spread over a large area, its contact pressure might actually be less than a much lighter roller with a smaller contact area. Look at it this way, would you prefer a 16 stone bloke in boots to tread on your foot or an 8 stone woman in stilettos? (You don't have to answer that one!)
Remember: Pressure = Force / Area Contact Area:
So, how do you calculate contact area? Ideally, this needs to be done on hard- standing so that you have a value that is comparable between machines (if you measure one roller on a soft surface the roller will sink in a little, giving you a higher contact area than a comparable machine would when measured on a hard surface). Measure the length of the drum in contact with the ground (and the other 1 or 2 if you have a tandem or Tri - roller). The next bit is tricky and will require being on hands and knees, as you need a measure of the width of the drum in direct contact with the floor. Hopefully, after this, you will have, for a tandem roller, something like this:
Front Drum: Length 1.5m
Rear Drum: Length 1.5m
Contact width 0.08m Contact width 0.08m
To get the contact area just multiply the length by the contact width For both drums above, the contact area is
roller as a whole the contact area is 0.24m2
1.5 x 0.08 = 0.12m2 (2 x 0.12m2
Contact Pressure:
In the case of this example therefore, the weight of the roller is spread over 0.24m2
To turn this into a contact pressure we need to know the force exerted by the roller (measured in Newtons - symbol N), or for easier calculation, kilo-Newtons - kN).
Physics states that: Force = Mass x Acceleration
It’s strange to think, but all of us are subject to acceleration due to gravity - that’s what holds us on the planet, the same applies to the roller. So, for the example above, if the roller’s mass is 1600 kg, then the force is mass x acceleration due to gravity (9.81 m/s2
Force =Mass x Acceleration =1600kg x 9.81m/s2 =15696N =15.7kN
To make this kilo-Newtons just divide by 1000, we’ve rounded the number up here to 15.7 kN
Remember: Pressure = Force / Area
So then, the contact pressure for this roller is:
Pressure = Force / Area = 15.7kN / 0.24m2 = 65.4 kN/m2
A bit long winded I know, but now you have a figure against which you can make a meaningful comparison between machines. The contact area of the roller is key here and the larger the diameter of the drum, the greater the contact area. Interestingly, as a surface becomes harder through rolling, the contact area between the roller and the ground becomes less (as the roller does not sink in as much), therefore the effective pressure increases on the ground, further compressing it and making it even harder until the soil reaches its compressive limit and will not compact any further. At this point it is time to stop rolling, as you are achieving nothing else of benefit.
So, knowing the contact pressure of the rollers will enable you to choose a roller
. ). , therefore for the
with the greatest contact pressure of those available, regardless of their weight! Clearly, if a roller you are considering is variable weight (through the addition or removal of water ballast), then you need to calculate contact pressure at the weight you plan to roll at. As a useful guide, 1 litre of water has a mass of approximately 1 kg.
Other issues in respect to effectiveness of the roller will be minimum travelling speed - the slower the speed the greater the contact time and the more effective in terms of compression each pass will be. It is likely, albeit unproven at present, that a slower speed is less likely to cause problems with fracturing of soils by the pressure wave ahead of the roller as it moves.
) so:
Similarly, the diameter of the roller affects the angle of contact with the soil and this might also influence the disruptive effect roller. So, while it might be tempting to get more contact pressure by reducing the roller diameter (giving you a smaller contact area), this could have disastrous effects on your pitches. If you desire more contact pressure, it is better to add more mass to a larger diameter roller, e.g. by adding ballast.
To help answer these questions (and many more) Cranfield University has trials underway, sponsored by the ECB, looking at the optimal moisture content at which to roll, the effect of different moisture contents in different layers in the soil on the creation of root breaks, and the effect of different roller diameters on the pressure bow-wave produced. How this bow-wave deforms and moves soil in front of the roller at different moisture contents and strengths will also be looked at. The results of this work will be coming out over the next few years.
As regards the mechanical aspects of roller choice I defer to the mechanical experts amongst you for guidance and, clearly, any machine you choose should be safe. Above all, when rolling it is vital to use safe operating procedures and never leaving the roller whilst it is in motion or the engine is running.
Happy rolling!
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