Ian Mitchell, a former Senior Training Technician at Cutting Edge Training, now runs his own training company. In this article for his former employer, he discusses the issue of vibration - how it is measured and its effect on operators working with outdoor power equipment
In the last article, we looked at noise, its implications and the way it is measured. In this issue, we are tackling a related subject and looking at vibration.
What is Vibration?
Vibrations (in turf machinery) are usually short, high frequency movements. They often have a set frequency which can cause mechanical wear and damage. This can result in metal fatigue, the movement of clamped components, the failure of seals and bearings, noise generation and, of course, possible discomfort to the operator.
Where do vibrations come from?
Vibration sources are many and varied, but the common causes are often engines and motors, exhaust systems, hydraulic systems, drive shafts and belts, gearboxes and transmission units, cutting units and rotary decks, tyres and rollers and the surface you are working on, or travelling over.
Noise is a vibration which has travelled through the air and reached your ears. The intensity and frequency of the vibration will determine what you actually hear.
Well, some of our machinery deliberately create vibrations in order to achieve an end result; vibrating rollers and some aerators are a typical example. Other machinery, like deep tine aerators, create considerable movement, or shaking due to the way they work (large rotating crank shafts and lots of fast moving parts). But, unless the vibration is a deliberate end result of using the machine, then most vibration is, in fact, bad and should be avoided if possible.
Vibration increases soil compaction, it reduces the working life of machinery and equipment, or causes manufacturers to build heavier and, therefore, more expensive machinery to cope with, or absorb, the vibrations created by using it.
The measurement of vibration
Vibration is not good for the operator, and all vibration needs to be measured with exposure levels monitored and recorded. Vibration is usually recorded in two different categories,
- HAV (hand and arm vibrations)
- WBV (whole body vibration)
The unit of measurement is m/s² (metres per second squared). Basically, a sensitive measuring device is attached to the part of the machine to be tested, usually the handles or the seat, and any movement is recorded. Movements will be very small, but very fast. The measuring device, or meter, will record the movement and give out a reading in m/s2. The legal daily limit for vibration exposure is 2.5m/s².
The Operator's book
There is a clear duty under CE certification for manufacturers to include the vibration data in the Operator's Instruction book. A ride-on machine will have to show its HAV values (steering wheel) and WBV (seat and footplate area). Handheld machinery will include HAV values only.
The chart, taken from a Jacobsen GP400 Operator's Manual, shows the way the vibration values are shown in the operator's instruction book, and quotes the relevant EN standards to which the machine was tested.
The effects of vibration
Vibrations damage the nerves in our skin, and any areas of the body which come into contact with the vibrations can be effected. Nerve damage can result in the loss of touch and sensation, making everyday tasks like doing up buttons more difficult. More extreme damage can result in intense pain during cold weather and severe dexterity problems.
With many powered hand tools, the measured HAV reading will be above the 2.5m/s² limit, so the only way to stop exceeding the legal limit is to reduce the actual usage, or trigger time. 2.5m/s² is the daily max, taking eight hours as a normal working day.
So, if a piece of machinery registered 5m/s², the total run time for that operator has to be reduced to four hours, giving the same exposure as eight hours at 2.5m/s².
Given the requirement for breaks, refuelling, changing cord and moving around on site, very few operators will achieve more than three or four hours trigger time in a normal working day.
For self-propelled machinery, the operator is sitting in the seat all day, so the exposure times are going to be much higher. Thankfully, the vibration levels on bigger machinery are very much lower, and the majority can be used all day, every day without any concern about vibration exposure levels.
The best way to protect against vibration damage is to keep the affected areas of the body warm. This improves blood circulation and reduces the effect of vibration.
So wear gloves whenever you are using vibrating machinery; they really can make a difference.
A popular, and very cost effective way to identify machinery for operators is to run a traffic light system.
- Red tape on the handles means two hours use only per day
- Yellow tape on the handles means half day operation
- Green tape on the handles means all day operation
Alternative systems give each operator a vibration reader, and then total vibrations for each day and week can be very accurately monitored and recorded.
The right solution has to depend on the expected level of risk. Coloured handles are okay, but if the operators are close to maximum exposure levels all the time, then a more sophisticated system may be more appropriate.
Reducing vibration levels
So far, we have looked at what vibrations are, how they are caused, their measurement and the effects they can have on us. We should now look at what we can do to reduce vibration on the machinery we own and operate.
Get it serviced!
The machinery left the factory in perfect condition - new bearings, balanced shafts and pulleys, good exhaust systems and seals etc. As machinery is used, it wears and may suffer damage. As an end user, you can't do much to reduce the built-in vibration levels, but you can assist in maintaining them to the quoted values.
A thorough service should identify damaged parts and they should be replaced. Worn bearings and slack drive belts create additional vibrations which pass wear and metal fatigue to other components. Remember to check all chain tensions as well, and then reset them as per the service manual. Over tensioning can create as many problems as too loose.
Lubrication (grease nipples) is often missed by operators, and these do need attention. Manufacturers are working to remove as many as they can, but some are simply essential. Make sure they are being regularly lubricated.
Blades and Cylinders
You should always fit genuine parts, as they are designed for a precise fit and made to tighter tolerances. Even with genuine parts, you should always check that rotary blades are balanced before fitting them. Cylinders are also critical to smooth vibration free operation.
Genuine parts are dynamically balanced, look for signs of balancing makes or weights on any non-genuine parts you may have, they are cheaper for a reason!
If you have sharpened a blade, then rebalancing is essential. Use a quality balancer to get an accurate result. The operator and the rest of the machine will notice the difference, even if you don't.
All manufacturers operate in a competitive world and are constantly working to produce quieter, more comfortable machinery, not just to meet legislative requirements, but because they know that the operator's feedback and driving experience are key deciding factors in the purchase of new machinery.
So, smoother transmission systems, quieter hydraulics and now hybrid drive systems are being introduced, all of which contribute to a better driving experience. Electric motors generate much less vibration than engines, so battery powered chainsaws and hedgecutters really do make a difference. Hybrid ride-on machinery is much smoother as well, bringing noise and vibration reductions with it.
Some low tech solutions are also being used, like replacing chains with toothed drive belts for smoother and quieter drives.
Factory fitted cab systems are now much better, and the isolation mountings under the operator's platforms help to remove vibration from the driver.
So manufacturers are working to produce machinery with lower noise and vibration levels. The actual operator's exposure levels have to be recorded in the instruction book, and the operator's exposure to vibration has to be measured and monitored to ensure the legal limits are not exceeded.
On existing machinery, a full servicing programme and replacement of worn or damaged parts can all contribute towards reducing the exposure to harmful levels of vibration at work. And the easiest thing operators can do to reduce the effect of vibrations is to take regular breaks and to wear gloves when using vibrating machinery.