How ram mounting geometry affects safety
The three main dimensions
Gate ram mounting geometry can be described in terms of three basic measurements: X, Y & Z.
The X and Y dimensions describe the position of the gate pivot and ram pivot relative to each other rather than
relative to the post. The ideal geometry for ram mounting is where X and Y are equal dimensions as the leverage
and mechanical advantage in all phases of opening and closing are equal. The Z dimension is dictated purely by the
overall length of the ram.
Where X and Y are equal there will be the least stress on:
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the ram itself
the hinges and hinge mountings
the posts, the post foundations and the ram mounting brackets
the gate structure
It should be noted at this point that hinge failure and hinge mounting failure are a common cause of gate related
accidents.
The ideal dimension for X and Y is actually half the operating stroke of the ram.
The usable operating stroke is the actual stroke less 8 – 10mm at each end of travel to prevent the ram coming to
the end of its stroke. This is referred to as end stroke prevention and is commonly required by ram manufacturers.
For example:
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Actual ram stroke = 290mm
Usable operating stroke = 270mm
Ideal X and Y = 135mm x 135mm (270 ÷ 2)
This will give 90° of leaf travel
By reducing X and Y by 10%, we will achieve the maximum gate opening which will be in the order of 120°.
If these dimensions are reduced further, all that will happen is that not all of the ram stroke will be used, gate speed
will increase and performance will be reduced.
So, even if the ram manufacturer’s data not is not available, a usable X and Y dimension can be derived by measuring
the operating stroke of the ram and halving it. This can prove very useful when attempting to work out why an
existing system of unknown provenance may not be working as well as expected or, indeed, failing force tests.
Fast and slow phases
When X and Y are not equal, the gate leaf speed will not be constant relative to ram speed throughout travel; there
will be a faster and a slower phase of leaf movement.
During a slow phase, the ram will exert more force; this will increase the force applied by the gate to an obstacle or
person.
During a fast phase, the gate will be carrying more inertia; this will increase the initial impact force on an obstacle or
person and negatively affect safety system reaction time. The ram will have problems moving the gate from
stationary; this effect can be likened to a car attempting to pull away in 3rd gear. Hence, a safety activation during a
fast phase will mean that the gate has to be stopped from high speed and then sent back in the opposite direction
using too high a gear. This can, of course, be mitigated by using ever more powerful rams but this in turn applies
additional stress to the system as a whole.
Ram to post clearance
The “Y” dimension minus the “Y1” dimension must equal more than half the ram case width to ensure there is
adequate clearance between the post and the ram casing in the closed position in order to prevent finger trapping.
As X and Y are dependent on ram stroke, a long stroke ram may well better suit a large post diameter due to its
inherently greater X and Y dimensions.
Ram to gate angle
The gate leaf centre line is assumed to be on the hinge centre line, hence any significant difference must be
accommodated with packing/spacers etc at the gate attachment bracket to maintain the correct ram angle.
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Post bracket reinforcement
As the ram pivot is always located relative to the gate hinge centre, “back hung” gates will result in long post
brackets so care will be needed to ensure they are effectively braced to withstand forces generated by motor and
leaf. Any flex in the mountings will have a negative effect on safety system reaction times.
Wind affected and fully boarded gates
Although all swing gates are affected by wind to some degree, once the amount of infill goes beyond 30%, the
effects become much more dramatic. The use of rams with inherent obstacle detection will give rise to unreliability
in use, as the wind effect will be seen by the system as an obstacle and retracts. So, for reliable service, the use of
non-detecting rams is usually preferable. Additional power will be required to move the gate reliably against the
wind. This additional power can also be utilised to provide safety, as the same high power will be available to effect a
retraction. This will however require the use of deep section safe edges to combat high initial impact forces and
provide acceptable reaction times.
It must also be appreciated that a wind affected gate will need greatly reinforced posts, hinges and ram mountings in
order to resist these increased forces. Any flexibility in the system will reduce reaction time to safe edge activation,
which will lead to excessive initial impact forces to quite dramatic levels. The use of good geometry will assist
greatly in controlling these potentially dangerous forces. A considerable safety factor will need to be provided and,
for this reason, DHF recommend a maximum of 200N in calm conditions.
Conclusion
Care in design and selection of components, together with correct installation and adjustment, will result in less
stress and more consistent movement. The gate will also respond better to safety device inputs and produce better
limitation of force.
It will also be more reliable!
©Door and Hardware Federation, Powered Gate Group, 42 Heath Street, Tamworth, B79 7JH. December 2014
Office: +44 (0)1827 52 337 Website: www.dhfonline.org.uk Email: [email protected]
DHF 1087:01/15
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