Contents
Limit Switch
Planning
1. Function
Limit switches are primarily used in Screw Jacks and electromechanical lifting cylinders to restrict or stop the lifting movement, preventing the lifting elements from reaching a critical position. Additionally, limit switches can be used for position detection, but they only provide a binary signal (0 or 1). For continuous position monitoring, encoders or potentiometers are more suitable.
Screw Jack with limit switches
2. Types of Limit Switches
In Screw Jacks, three types of limit switches are typically used: inductive limit switches, mechanical limit switches, and magnetic switches.
Inductive Limit Switches
An inductive limit switch works by detecting changes in the electromagnetic field caused by the presence of a metallic object. The switch generates an electromagnetic field around its sensing area. When a metallic object enters this field, the field changes. This change is detected by the switch and triggers an electrical signal.
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Contactless and therefore free from wear. However, they don't respond to all materials.
Mechanical Limit Switches
A mechanical limit switch works by being physically activated by a moving part of the machine. When the spindle or another moving part reaches a certain position, it presses on the switch, closing or opening an electrical contact. This contact sends a signal.
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Robust and reliable, but can be affected by dirt and wear.
Magnetic Switches
A magnetic switch works by responding to the presence of a magnetic field. When a moving part of the machine containing a magnet comes close to the switch, a reed contact inside the switch is opened or closed by the magnetic field. This contact then sends an electrical signal.
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It also works through non-magnetic tubes, but a triggering magnet is always required.
3. Limit Switches on Screw Jacks
Screw Jacks with a lifting spindle (basic design) can be directly equipped with corresponding end-of-travel or intermediate position limit switches. For this purpose, a hole or slot is made in the protective tube, and an adapter is attached to the tube. The thread for limit switches is usually M12x1 or M18x1. The limit switches can either be set to a fixed position or prepared for a variable adjustable position. For mechanical and inductive limit switches, the standard adjustment range is +/- 5mm.
Magnetic switches, since they don't require a hole in the protective tube, can be continuously moved along the entire length of the protective tube.
Screw Jacks with a rotating spindle and traveling nut (traveling nut version) cannot be equipped with their own end position sensors. The limit switches must be mounted on the lifting frame.
4. Limit Switches on Electric Cylinders (EZ)
Electric Cylinders EZ are standardly equipped with mechanical limit switches. These switches are adjustable to some extent via threaded rods on the lifting profile. Additionally, mechanical safety limit switches can be installed for increased reliability. For seamless adjustment along the stroke path, magnetic switches can also be integrated into the aluminum profile.
Electric Cylinder with limit switches
5. Limit Switches on MINI Actuators
MINI Cylinders are generally equipped with end position and safety switches. End position adjustment is only possible for the retracted direction using a set ring. The limit switches can be operated via an external controller or wired internally within the cylinder to enable automatic motor shutdown.
MINI Actuator with limit switches
6. When to use Limit Switches?
End position switches should be used in every system to limit stroke movement. Unexpected overrunning of the end positions often leads to mechanical damage to the system or lifting element. Even when encoders or other control units are used, the end positions should always be connected to the stroke control as a first step.
Limit switches can also be used for intermediate positions, but they must be tested for both the retracting and extending directions, as the switching cam itself needs to be accounted for.
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