Pallet lift
Pallet lifts are a special type of lift table, which is specialised in lifting and lowering of Euro or industrial pallets. This design is often used as a pallet loading station in front of (partially) automated production lines.
In most cases, flat lift tables with a special platform geometry are used as the basis for the pallet lifts. The design of the platform allows the lift table to be loaded and unloaded using a simple pallet truck. This eliminates the need for expensive forklifts.
Passenger lift
This type of lift is designed for the transport of people or for the transport of people and goods.
Payload
In the case of a lift table, the payload corresponds to the permitted maximum load-bearing capacity of the lift table. The payload is specified in a unit of weight (e.g. kilograms or tons).
A lift table must be capable of lifting not only the payload but also the weight of the platform and approximately half the weight of the scissor lift mechanism.
Pendulum volume
The term pendulum volume is used in connection with double-acting cylinders. It is the difference between the amount of oil required when the cylinder retracts and the amount of oil required when the cylinder extends. Thus the pendulum volume equals the volume of the piston rod.
Double acting cylinders have two chambers, each of which can be pressurised. Whether the cylinder retracts or extends is determined by which chamber is filled. The filling of one chamber always causes the other side to be emptied.
This outflowing oil does not have to flow back into the tank, but can be used to fill the other chamber. However, the oil quantities required to retract and extend the cylinder are not equal, as part of the available space is already occupied by the piston rod during retraction.
This means that more oil is needed to extend the cylinder than to retract it. The oil in one chamber is therefore not sufficient to allow the cylinder to extend completely; additional oil must be pumped from the tank into the cylinders. When the cylinder is retracted, there is again more oil than is needed for the corresponding chamber. So the oil flows back into the tank. The oil therefore constantly oscillates between the tank and the cylinder when the cylinder is retracted and extended.
As the pendulum volume must be provided from the tank, it serves as a size for dimensioning the tank volume.
Pipe rupture safety valve
Pipe rupture safety valves are designed to prevent the platform from falling off a lift table in the event of a hose or pipe rupture.
Pipe rupture safety valves are activated when a certain pressure difference between cylinder and connection line is exceeded. If a hose ruptures, the safety device is exposed on one side to the operating pressure in the cylinders, while on the other side only the atmospheric ambient pressure has a counteracting effect. If the pressure difference and so also the volume flow between the two sides of the safety device is high enough, a valve plate closes the outlet openings and prevents the hydraulic cylinders from retracting further.
Due to this operating principle, the pipe burst safety valves must not be set too close to the actual operating flow rate. Otherwise the safety device would be triggered when the lowering valve is opened and prevent the platform from being lowered. As a rule, the pipe rupture safety valve is set to about 150 % of the maximum normal flow rate.
Furthermore, the pipe rupture safety valves are not activated if there is a minor leak in the pressure line. A leak causes only a small pressure difference between supply line and cylinder, so that the response pressure of the pipe rupture safety valves is not reached.
Electrically or hydraulically pilot-operated valves is recommended when special requirements are placed on the safety of a scissor lift table . In the basic position, they prevent the hydraulic cylinders from retracting. The lifting platform can only be lowered when an electrical voltage is applied or when the control line of the valves is pressurised. This guarantees maximum protection, also against leaks.
Piston rod
The piston rod is the moving part of a hydraulic cylinder. The movement of the piston rod generates the up and down movement of the platform in a scissor lift. In other words, The annular piston side of the piston rod must be exposed to pressure sufficient to accelerate the load to be lifted.
Plant lift table
Synonym for plant lifting platform.
Plant lifting platform
In the lift table industry, the term plant lifting platform has become a synonym for lift tables with high payloads and frequent lifting cycles.
This is due to the high demands in a manufacturing 'plant'. If the lift table is part of an automated production line, short cycle times often have to be implemented in order to increase output. These high demands are met with appropriate equipment features (e.g. with bearings that can be lubricated, more robust mechanics, etc.).
Platform
The load-handling device of a lift table is called a platform.
The design ranges from simple rectangular surfaces to customised platform geometries with extension platforms.
Polarised light barrier
The term polarised light barrier is used for reflective light barriers. In reflective light barriers, the transmitter and receiver are located in one housing. The light beam of the transmitter is reflected on the opposite side back to the receiver. The light barrier detects when an object interrupts the light beam.
However, the disadvantage of simple reflective light barriers is that it does not distinguish by which object the light beam is reflected. Simple reflective light barriers may therefore not be used if there is a risk that a shiny metal surface could reflect the light beam, for example.
Polarised reflective light barriers are available for precisely these applications. The transmitter only registers the light beam when it is reflected by a retroreflector. A reflection on a shiny surface is reliably detected as an interruption of the light barrier.
Positive form locking
In positive form locking, the force between several components is transmitted through their shape or geometry. Good examples of a connection in which the transmission of force is based on the principle of positive form locking are dovetail joints, feather keys or tongue and groove joints.
Potential-free contact
A potential-free contact is used for the simple transmission of digital signals between two independent components. The circuits of the different controls are galvanically isolated from each other. The great advantage of this is that no attention needs to be paid to the polarity of the contacts, the correct earthing, the correct type of voltage and current.
In a scissor lift table , potential-free contacts are used in particular when signals from the scissor lift have to be transmitted to a higher-level plant control or a building management system. For this purpose, the lift table control system switches a relay, for example. The output side of this relay is in turn integrated in the higher-level plant control. Operating states can be transmitted in a simple manner through this kind of coupling.
Power reserves
The power reserve is the buffer between the actual (higher) output of a machine and the required output.
While the lifting and lowering time of a lift table usually does not provide for any power buffers, the working load limit (WLL) of the scissor lift table is generously dimensioned by some lift table manufacturers in order to rule out a mechanical failure of the scissor lift.
Power unit (aggregate)
Word origin: from Latin aggregare = to add
In the technical field, a combination of several coupled individual machines or apparatuses is referred to as an aggregate.
In the case of a hydraulic scissor lift table , the power unit (aggregate) refers to the drive unit of the lift platform. The drive unit consists of the following individual parts: Electric motor, coupling, hydraulic pump, tank and valve block. If the term "power unit" is taken very broadly, the electrical control unit, hydraulic cylinders and wiring can also be listed as components.
Pre-lifting limit switch
The pre-lifting limit switch signals the lift table controller that the platform will soon reach the top end of the lift. The lifting speed of the lift table is usually throttled from the moment the pre-lifting limit switch is actuated to ensure that the top end position is approached smoothly.
In comparatively slow lifting tables, the installation of a pre-lifting limit switch is not necessary.
Pre-lowering limit switch
The pre-lowering limit switch signals the control unit that the lift table platform will soon reach the bottom end of the lift. The lowering speed of the lift table is usually throttled from the moment the pre-lowering limit switch is actuated to ensure that the bottom end position is approached smoothly.
In comparatively slow lowering speeds, the use of a pre-lowering limit switch is not necessary.
Press-fit
In case of press-fit (frictional connection), the force is transmitted between several components with the aid of static friction.
One of the best known examples of this is the connection between a tyre and the road. The vehicle can accelerate if the driving force is less than the static friction between the tyre and the road. If the driving force exceeds the static friction, the powered wheels start to spin. The vehicle does not accelerate. Only an increase in friction (e.g. due to an increase in tyre temperature) or a reduction in driving force will cause the vehicle to move forward.
Other examples of a frictional connection are knots, slip friction clutch and screw connections.
Pressure relief valve
Pressure relief valves can be set to a maximum pressure and protect the other components of the hydraulic circuit against impermissibly high operating pressure.
The permitted pressure is usually set by the preload force of a spring. This spring counteracts the pressure in the hydraulic circuit. If the set pressure is exceeded, the operating pressure compresses the spring and some hydraulic oil can get into the hydraulic tank from the pressure line. As soon as the operating pressure falls below the set pressure again, the spring closes the valve again.
In simplified goods lifts and lift tables, the pressure relief valve is set so that the maximum permitted payload can still be lifted. This protects not only the hydraulic but also the mechanical components from excessive stress. When the lift or the lift table can also be loaded when raised, an additional device for pressure measurement is necessary so that lowering with a too heavy load can also be prevented.
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