DHOLLANDIA tail lifts, a complete range of tail lifts
5
Automatic tilt at work
Main objectives
•
Explore and understand the functional principles behind automatic tilt at ground level
•
Define the differences between hydraulic & mechanical automatic tilt at ground level,
and understand the practical implications
•
Know how to advise a customer to use automatic tilt correctly
•
Evaluate and understand the importance of the load diagrams
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1. General logic
•
What makes the platform stay level, although it makes a rotational movement up & down?
•
Once the platform is in horizontal position at the ground, what can make the point tilt down to
the ground?
mechanical tilting = increasing length of lift arm
hydraulical tilting = decreasing lenght of tilt cylinder
2. Hydraulic tilt at ground level: intuitive understanding
LC = lift cylinder
TC = tilt cylinder
M = hydraulic memory
Start assumption:
Platform stands at loading floor,
Ready to lower
Pressure determined
by nominal weight on
platform (primarily)
Hydraulic memory
Pressure determined
by position of weight
on platform (primarily)
•
•
The hydraulic auto-tilt works on the basis of a hydraulic memory cylinder that links the lift cylinders LC with the tilt
cylinders TC.
When the platform touches the ground during the downward movement, the tilt cylinders TC contract and the
platform point inclines to the ground when the function ‘lower’ remains activated.
STEP 0 : opening the platform
STEP 1 : the platform is at floor level, and is lowered down to the
ground.
Basic formula: P = F / A
P = Pressure
F = Force
A = Surface
Assumption X1: when load is correctly
on centre of gravity, PLC = PTC.
Assumption X2: hydraulic memory is
manufactured so that
A(M)LC = 2 A(M)TC
So: P(M)LC = F(M)LC / A(M)LC (1)
and P(M)TC = F(M)TC / A(M)TC (2)
Under assumption X1: (1) = (2)
F(M)LC / A(M)LC = F(M)TC / A(M)TC
Result:
F(M)LC = 2 * F(M)TC
As F(M)LC > F(M)TC, the oil first flows
out of the LC, and the platform lowers
to the ground with the point remaining
horizontal.
STEP 2 : when arriving at ground level, the platform point tilts
down
Platform rollers touch ground
Result: FLC  0
The oil flows out of the LC-side of
the hydraulic M, and is filled with oil
from the TC, because F(M)TC >
F(M)LC.
This contraction of the TC
automatically makes the platform
incline to the ground.
STEP 3 : Lift sequence: the platform point is tilted up to horizontal
position
When pump turns & pressure
builds up:
F(M)LC = F(M)TC * 2 or
F(M)LC / 2 = F(M)TC
It takes ½ the force to close or tilt
up the platform than it takes to lift
the platform.
Oil from the pump pushes the
piston inside the memory M back
to right side, and oil on TC-side of
the M cylinder is pushed back into
the TC, pushing the platform up to
horizontal position.
STEP 4 : When the platform is back at horizontal level, it
continues to lift further to the loading floor
When the M cylinder is shifted to
the right and end of stroke, the oil
continues to flow into the LC, and
the platform lifts up at a horizontal
angle to the loading floor.
Remark: importance
diagrams.
of
observing
load
As the pressure in the tilt cylinders TC is primarily
determined by the position of the weight of the platform,
moving out the load 2x or 3x the distance of centre of
gravity towards the platform point, will increase the
pressure in the TC to 2 * PTC or 3 * PTC.
If we then compare F(M)LC with F(M)TC:
F(M)LC = P(M)LC * A(M)LC
And
F(M)TC = P(M)LC * A(M)TC with P(M)TC = 3 * P(M)LC
Then:
F(M)LC = P(M)LC * 2 A(M)TC = 2 * P(M)LC * A(M)TC
And
F(M)TC = A(M)TC * 3 * P(M)LC = 3 * P(M)LC * A(M)TC
So:
F(M)TC > F(M)LC !!!!
Thus when the centre of gravity point of the load is too far
away to the point of the platform, the force-couples
reverse.
The point inclines before the platform lowers to the
ground !!!
3. Importance of obeying load diagrams
4. Mechanical tilt at groundlevel: intuitive understanding
Start assumption:
Platform at loading floor,
Ready to descend
LC = lift cylinder
TC = tilt cylinder
TC is fixed, plays no role!
The tilt bracket remains tight against the lift frame,
and the platform remains horizontal as long as:
FLC * B > FA * A
Forces on mechanical tilt bracket:
1
2
The mechanical auto-tilt is operated by a lever or “mechanical tilt bracket”, that articulates on the lift frame, and holds
the lift arm and lift cylinders.
Contrary to the hydraulic auto-tilt, the platform doesn’t tilt its point to the ground by a compression of the tilt cylinders
TC, but by a further compression of the lift cylinders LC when the platform reaches the ground, and the function
“lowered” remains activated.
STEP 1 : the platform is at floor level, and is lowered down to the
ground.
When the load is on correct centre of gravity on
the platform, the force exercised by the lift
cylinders FLC over the leverage distance B, is
larger than the rotating force FA exercised by the
platform and its load over the leverage distance
A.
FLC * B > FA * A
The lift cylinders stroke in, the tilt bracket remains
tight against the lift frame, and the platform
lowers to the ground with platform in horizontal
position.
STEP 2 : when arriving at ground level, the platform point tilts
down
Platform rollers touch ground
Result: PLC  0 or FLC  0
therefore:
FLC * B < FA * A
Important: tilt cylinders TC are fixed and don’t
change position!
The proper weight of the platform and the load
pulls on the lift arms, makes the tilt bracket
swing away from the lift frame, and force the lift
cylinders (left without pressure) to stroke
further in.
The tilt cylinder being a fixed point, and the lift
cylinder extending out, the platform rotates,
and the point inclines down to the ground.
STEP 3 : Lift sequence: the platform point is tilted up to horizontal
position
When pump turns & pressure
builds up:
FLC * B > FA * A
Assumption: B = 2 *A
It takes ½ the force to close or
tilt up the platform than it takes
to lift the platform.
The pressure in the lift cylinders LC push the tilt
bracket back against the lift frame, and the lift
arm is pulled back towards the lift frame (or lift
arm is “shortened”.
The tilt cylinder being a fixed point, and the lift
cylinder shortening in, the platform rotates up,
and the point lifts up to horizontal position.
STEP 4 : When the platform is back at
horizontal level, it continues to lift
further to the loading floor
The platform continues to lift up
at a horizontal angle to the
loading floor.