The commercial grass cutting business mainly works within the urban landscape, with grass machinery working on housing estates, roadside verges, parks, playing fields and schools. As these areas are all man made and are in fact "designed spaces" for easy living, it would seem obvious that the maintenance of the open spaces would also have been carefully thought about, but perhaps not. So often, the grassed areas are used to blend different levels together, and they can include some awkward banks and slopes, which must be maintained.
There are very few naturally flat sites and, even then, some banks and slopes are specifically designed to enhance the landscape or control noise pollution, so we often have large areas of grass covered slopes to maintain.
So, having machinery which can work on sloping ground is important, as well as having risk assessments and working practices which prevent machinery from being operated beyond its safe working limit.
In this article, we look at the design constraints of modern machinery and the ways in which manufacturers are using technology to enhance performance and safety.
The demands of the job have led to the creation of the modern triple mower (or out-front rotary) design. These combine the manoeuvrability required to get around the sites, with the work output to keep up with demand. Compromises have to be made along the way, and one of these is the requirement for a narrow wheel base machine (to get into tight areas and through gateways) whilst, at the same time, retaining the ability to work on sloping ground and banks.
There is also the operator to think about. All ride-on machines now have a ROPS frame fitted as standard. This provides a safer working environment for the operator, which is particularly important when working on sloping ground. Ironically, fitting a steel ROPS frame to the machine increases the weight, and it raises the centre of gravity which actually reduces the safe working angle of the machine, despite giving better protection to the operator.
Fitting a cab also impacts slope mowing performance by raising the centre of gravity and the weight of the machine.
Centre of Gravity
The stability of a machine is governed by the width of the wheel base and the height of the machine. Tall and narrow machines are inherently less stable than low and wide ones. However, as mentioned above, the other demands of the job require a narrow wheel base, so stability is a real issue. The engine is the heaviest single part of the machine, so this should really be as low as possible - all perfectly sensible - until you start to climb curbs and need to have higher ground clearance. The image shows the effects of the centre of gravity (CoG) on a machine, and how this sets the limit for any machine when working on sloping ground.
As the slope angle increases, the line will move closer to the outside edge of the downhill wheel. Once the centre of gravity line moves outside the wheelbase, the machine will become unstable and could easily roll over.
Safe Working Angle
Every machine should display a safe working angle. On triple mowers and out-front rotaries, this is usually 15 or 16 degrees, or something very similar. Any experienced operator will be able to use the machines at higher angles than this (where ground conditions allow), and many therefore disregard the working angle. So, it's probably important to understand how this working angle is achieved and what it actually means.
EN Standards: there is a European national standard and legislation covering the stability testing of machinery; this is known as EN 836. Under this EN standard, the machine must be tested in its worst-case scenario.
So, to achieve the worst case, the units are usually raised (to increase the CoG), and the machine slowly tipped over on a table to see how far it will go until a wheel starts to lift. A steel plate is placed under the wheel, with a 10N force pulling it. When the plate moves, the test is stopped and the maximum angle is recorded.
As this is a static test, the recorded angle is then halved to take into consideration other forces working on the machine, (like forward momentum and uneven ground), and to allow for a suitable margin for error.
So, a machine with a 15-degree safe working angle decal actually achieved 30 degrees on the static test. Clearly, a confident operator could, with the cutting units on the ground, easily exceed this theoretical limit but, when on steeper ground, any sudden dips or hollows in the surface could take them close to, or beyond the physical limit of the machine.
The other concern is the operator lifting the cutting units whilst still on a slope. Any attempt to lift the deck or units will change the centre of gravity and could dramatically affect the stability of the machine.
Tilt Stabilisation Technology (TST)
One way of improving the machines test results, without increasing the wheel base or redesigning the whole machine, is to limit the operator's ability to change the centre of gravity of the machine whilst it is on a slope.
As discussed in the above section on the EN836 standard, the tests are carried out with the units raised for the worst case scenario. However, if the operator cannot raise the units beyond a certain point, then the worst case scenario can be revisited, and will now be taken with the units on the ground, giving a more stable base and a correspondingly higher test result.
The more recently designed machinery has moved away from mechanical levers to control the hydraulic systems, and we now use switches of joy sticks to do the same job. This has led to more ergonomically designed working platforms for the operator. The switches and joy sticks are all connected to an electronic control box (an ECU), which then sends power to remotely located valves within the machine to control the flow of oil to lift rams and drive motors.
The ECU's we use are industry common devices, and are programmed to run multiple functions as standard. So, we can often do more with them than just running the basic machine functions.
By fitting an inclinometer under the seat, we can accurately measure the working angle of the machine at all times and limit what the operator can do at pre-set stages.
On a Ransomes Highway 3 for example, the system has the following controls features built in to it:
- 15 degrees: A warning buzzer sounds and the digital display shows the actual working angle
- 18 degrees: The lift controls are disengaged, so the operator cannot raise the units, but can continue to cut grass. They cannot be restarted until the machine gets back to 16 degrees or less
- 22 degrees: The drive to the cutters is stopped, and the uphill unit is raised off the ground to give additional stability to the machine
By fitting such a system, the safe working limit has been raised from 15 degrees to 22 degrees, giving a significant increase in the safe performance of the machine, and limiting what the bolder operators are capable of attempting, therefore preventing accidents caused by overconfidence.
The system can be modified to work with out-front rotaries as well by not allowing the operator to raise the mowing deck and, finally, by disengaging the drive to the deck.
TST has proven that technology can be used to enhance machinery performance, whilst at the same time improving operator safety. When the cutter drive is automatically disengaged, the operator has little choice but to leave the area and look for flatter ground. By limiting the lift and lower operations, the operator cannot change the CoG of the machine while still on sloping ground. This significantly enhances the safety of the machine.
So far, we have looked at the mechanical limits that are built into a modern machine to assist the operator and prevent operation on steeper slopes. However, we all know that there is more to safe operation than just limiting machinery functions; even slight slopes can be hazardous in poor conditions.
The operator needs to understand the hazards associated with working on banks and slopes and how to assess the risks. Whilst a lot of this comes with experience, good basic training is also essential for safe operation.
The basic risk assessment questions need to be asked before attempting to work on sloping ground;
- How steep is the slope?
- What are the ground conditions like?
- What additional hazards are there on this site?
- Can I safely work on this bank today?
Operators must continually assess the ground they are operating on, and local knowledge of wet areas and poor soil conditions should be recorded on site specific risk assessments, and shared in site working instructions.
Banks and slope are part of our everyday working environment. By combining clever design and improving technology we can now safely work on areas which only expensive specialist machinery could safely work on previously.
DISCLAIMER: Machines and machinery manufacturers safety slope angles vary per model, so it is important to follow the safety instructions stated in the operator's manual.