SYNTHESIS STUDY FOR CINEMATIC SCISSOR LIFT TABLE SYSTEM

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THE SYNTHESIS STUDY
FOR CINEMATIC SYSTEM
SCISSOR LIFT TABLE

INTRODUCTION

This study derives from some analysis of type technical and commercial, following very
request of our clients for project electromechanical scissor lift table alternatively to those
which already exist on commercial.
The major issue of these systems is that to control the cinematic, which it isn’t of linear,
ie the actuators ( motors, pneumatic cylinders, oleodinamic cylinders, etc..) working by
linear mode on the contrary the scissors, for geometry they don’t respond with a linear
force. The issue don’t engrave only on tricky control position of the lift table, but also
for a choice of a size major of the power which it needs.
All the systems actual for scissor lift tables use belts or chain, these are all components
which they can have possible break, further at extensions by stress.
With the new system that we have created, it removes the use of belts and chains, and
besides it can install actuators of low power, and it gets a linear control with an accurate
control of lift speed and of height position.
In short, we can get one lifter as one of linear type leaving the advantage of use the
stability of scissor table.
Following, we make before a short analysis on actual scissor tables systems in
commerce, and after a mathematic compare between one lifter with a actual scissor and
the lifter with a scissor of new concept.

SOME ISSUE ANY MORE PECULIAR OF THE ACTUAL LIFTER WITH
SCISSORS

In the market there are different types of scissor lifts, following we do some example and
about technical considerations.
It is to be highlighted that all the systems used until now, have the same problem, that
require high strength in the initial phase of the lifting, then after decrease.
1. Hydraulic lifting type
This lifter is the traditional type, arises because the hydraulic cylinders can be inserted
inside of the scissors, and also in this position is imposed to deliver a very high strength,
so that the solution for the provision of the hydraulic force represents the best choice.

Idrauilic Scissor Lift Table

The issue that represents the system, are the following:
1) High initial effort;
2) High electromechanical power be installed on hydraulic power packs;
3) Vertical movement is not linear;
4) Industrial environments where it is not allowed the presence of oil on systems.

Lifting electromechanical belts type

This scissor lift has been designed as an alternative to those with hydraulic cylinders.
It was thought to insert an encoder on the motor to control the position in height, but
unfortunately also this system, while having an electric motor with inverter and encoder, is
ineffective for the correct positioning in height, as the linear advance of the motor does not
match to a linear advance of the lifting, as demonstrated below with a chart.

The issue that represents the system, are the following:
1) High effort to start lifting;
2) Installation of a power even 4 time greater than that required;
3) Installation of a higher power control;
4) High tensile stress on the belts;
5) No ratio of advancing motor and vertical lift.

Lifting electromechanical type rigid chain

This scissor lift has a very special characteristic, ie, the scissor has only the function of
guiding the table top, so it’s not the type carrier, while the lifting is done by a rigid chain
with links that are able to be stacked and become a pillar of steel.

With this system they are able to install motors of lower power than previous systems, and
it can have a linear control of the lifting with an encoder installed on the motor.
Unfortunately this type of system has three types of anomalies that make it decrease the
reliability:
1) The vertical chain undergoes a buckling stress that the section height ratio does not
meet the criteria of building science and the chain may deviate laterally and break
under load;
2) At the chain it requires an accumulation area when the table is low, and the greater
the stroke, the greater the amount of chain to be accumulated;
3) High noise during lifting and lowering, due to the connections that make up the
chain.

MATHEMATICAL COMPARISON, BETWEEN ELECTROMECHANICAL
SYSTEMS

Below it will be done a mathematical comparison, between a traditional system of lifting
pantograph and the system studied by us.
Technical data:
Sollevare un carico di 1000 kg con una corsa di sollevamento di 1500 mm in un tempo di
10 sec.
Diagrams of proportionality Motor / Lift
The first analysis was performed on the progress proportionality motor and its lifting, and
show the following results:

From the diagram it is
clear that in the case of
a traditional scissor lift,
the advancement of the
motor does not produce
a linear lifting ie at the
start the system is very
fast and further slows
down;

 

in the case
we studied, there is a
linear proportionality
between motor and
lifting stroke, as shown
in the diagram
opposite.

 

 

Diagrams of proportionality of the speed
Doing an analysis on the speed control it follows that traditional systems fail to have a
constant speed in lifting because the motors are applied to systems of levers that move
the lifting speed snapshots, so as shown in the diagram in a time of 10 sec. it starts with a
high speed up to slow down, and absolutely no in a linear mode.

In the system of new conception, the speed remains constant, without to look the phases
of start and stop.

Diagrams of proportionality of the lifting work
From the diagrams it is clear that the work did by the traditional scissor lift can not be
considered a work of the linear type but must speak of flash work, or the work required to
raise the stroke by an amount h

In the system of new conception, the work is constant, with the obvious considerations

Acceleration diagrams
As already noted in the diagrams of speed, the conventional systems do not have a
constant speed, for which you have to consider instantaneous accelerations, when
system varies the lifting speed for each infinitesimal, unless starting which imposes an
acceleration for to start stroke.

In the new system we designed with only the acceleration to start and the speed is
constant.

Power diagrams
As already noted in the work diagrams, the traditional systems do not express a constant,
whereby also the power required for to move is not linear and therefore must be installed
some motors high power especially in the starting phase from low position

Nel sistema di nuova concezione espressione di potenza è una costante proporzionale, di
cui si è sicuri del funzionamento della macchina.

In conclusion, based on the example shown, unless inertia typical of the system, we can
state the following data.

Traditional system:
1) Power resistant max 10.00 kW;
2) Variable speed no linear from 0.51 m/s to 0.15 m/s;
3) Spaces equal, did at different times;
4) No possibility of control by encoder installed on motor;
5) No constant of proportionality between motor and lifting stroke
New system:
1) Power resistant constant of 2.00 kW;
2) Average speed 0.15 m / s;
3) Spaces equal take equal times;
4) Ability to control the scissor lift with encoder installed on motor;
5) constant of proportionality between motor and lifting stroke.
In the end, the system conceived by us, not only allows the installation of lower powers,
but also allow the exact control of the position, since there are no transmission organs
deformable and no there are variations of the inconstant position.
Moreover, there is also a benefit from an energy point of view with the installation of
control systems of lower power.

 

 

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