Fork
lift trucks are designed to be safe when properly used. The
manufacturers all test their machines to the standards laid
down by one of three organisations and these standards are
the same in each case. More about stability testing can be
found by clicking here>>
To make a truck safe so that it doesn't overturn
in a forwards direction the manufacturers use a principle
very similar to that used in a child's see saw as anyone who
has been on an approved training course will tell you.
 Using
the see saw analogy we can see that in the first picture on
the left, the see saw is in balance. It will only be in balance
if the people on it are the same weight as each other and
also sit the same distance from the tipping point which is
known as the fulcrum. If, (say), the person on the right is
heavier than the person on the left then obviously the see
saw will tip over. This could be corrected if the person on
the left moved further away from the centre or fulcrum. It
is exactly this principle that is used in the design of a
fork lift.
As can be seen from this picture the load
is carried very close to the front drive axle whilst the
distance to the rear counterweight of the truck is longer.
This allows the counterweight at the rear to counterbalance
the load at the front with just a little bit "left over"
as a safety factor. It should be noted that the safety factor
is very small and you are advised to read the section on stability
testing to get a clearer picture. If you are a truck operator
don't be fooled into thinking that you have a lot of safety
factor - you don't!!!
Take another look at the picture on the right
where it says "Load Centre". This is arguably one
of the most important factors that an operator should be familiar
with before he/she attempts to pick up a heavy load. The standard
load centre nowadays is 500 mm (20 ins) as it is assumed that
a standard load will be 1000 mm. It was not always like this
and the original load centre in the UK was 24 inches (600
mm) assuming a 48 inch long load. I mention it because there
are still a great many trucks around with the 'old' load centre.
Some trucks even quote both load centres in an attempt to
assist the operator.
It follows from the above that every load
should be hard up against the front face of the forks, (often
erroneously called the heel of the forks), because any gap
here will increase the dimension on one side of our truck
see saw and it can tip over! A little trick I teach is as
follows: If you must have a gap between the load and the forks,
and some loads need to be undercut as it's known, estimate
the gap that will be created and mentally remove about 150
lbs for each inch of gap. In metric remove about 70 Kg for
each 25 mm of gap. An example follows:
 The
gap is about 100 mm - divide by 25 that's 4 'lots' of about
70 Kg to be removed so the truck will pick up approximately
280 Kg less than specified on the capacity plate. Note that
this trick will work on all trucks of all makes, counterbalanced
or reach, up to a basic capacity of about 5 tonnes. It may
work on higher capacities but I have not done the necessary
calculations to find out as trucks of 5 tons and less account
for 90% of the UK market. The picture on the left shows what
can happen if you overload the truck!
I'll finish this page by listing the things
that can cause the truck to overturn in a forward direction.
They are:
- Overloading the truck - Check the capacity plate
- Having a gap between the load and the forks as described
above
- Picking up a load that is longer than it should be
- Tilting the mast forwards when laden and elevated at
or near full height
- Driving too fast and hitting an obstacle that stops the
truck dead
- Incorrect truck operation on slopes and gradients
Apart from forward or longitudinal stability,
a truck can tip over sideways. This type of accident is even
more common and details of lateral stability can be found
by clicking here>>
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