How to Save Money When Buying Safety Limit Switch

Limit switches are essential devices in industrial applications that are designed to detect the presence or absence of an object or to monitor its position. They play a vital role in safeguarding machinery and equipment, preventing damage, and ensuring the smooth operation of manufacturing processes. There are various types of limit switches available, each with unique features and capabilities that suit specific applications. When choosing a limit switch, there are several factors to consider, which we will explore in this article.

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Environment

The operating environment is a crucial factor to consider when selecting a limit switch. The switch must be able to withstand the conditions of the environment without malfunctioning or failing prematurely. Consider factors such as temperature, humidity, vibration, shock, and exposure to corrosive or explosive substances.

For example, if the environment is dusty, a switch with a high level of protection against dust ingress would be necessary, such as an IP67-rated switch. In environments where there is a risk of explosion, a switch with an explosion-proof enclosure or a non-contact switch would be ideal.

Sealed Plunger Actuator Limit Switch

Contact type

There are two main types of contacts in limit switches: normally open (NO) and normally closed (NC). The contact type determines the state of the switch when no external force is applied. A normally open switch will have an open circuit when not activated, while a normally closed switch will have a closed circuit when not activated.

The choice of contact type will depend on the specific application. For example, if the switch is used in a safety circuit, a normally closed contact is usually the best choice because it provides a fail-safe condition if the switch fails.

Actuator type

The actuator is the part of the limit switch that physically interacts with the object being detected. There are various types of actuators, such as plungers, rollers, levers, and whisker actuators. The choice of actuator type will depend on the application and the position of the switch in relation to the object being detected.

For example, a roller actuator is suitable for detecting the presence of a box on a conveyor belt, while a whisker actuator is ideal for detecting the position of a robotic arm. In some cases, a customized actuator may be necessary to ensure proper detection.

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Stainless Steel Roller Limit Switch

Electrical ratings

Electrical ratings refer to the maximum voltage and current that the switch can handle. The ratings must be compatible with the electrical system in which the switch will be installed. Consider factors such as the voltage level, the type of current (AC or DC), and the maximum current draw.

If the electrical ratings are not compatible, the switch may fail prematurely, cause damage to other components, or create a safety hazard.

Switching speed

The switching speed is the time it takes for the switch to change state when the actuator is triggered. The switching speed is important in applications where rapid detection is necessary, such as in automated systems. It is measured in milliseconds, and the lower the switching speed, the faster the response time.

Mounting options

Limit switches can be mounted in various ways, such as surface-mounted, flush-mounted, or panel-mounted. The choice of mounting option will depend on the available space, the type of equipment, and the position of the switch in relation to the object being detected.

For example, a flush-mounted switch may be necessary if space is limited, while a panel-mounted switch may be ideal for applications where the switch must be mounted on a control panel.

Certifications and approvals

Depending on the application, the limit switch may need to meet specific certifications and approvals to ensure safety and compliance with regulations. For example, switches used in hazardous locations may need to be certified as explosion-proof or intrinsically safe.

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Hello!!

I have been asked to use a small (1/30 hp) worm drive gearmotor (12V DC) to rotate a discharge chute in response to operator input.

What is the best way to limit the maximum travel of the chute to 180 degrees? I know I could use limit switches/relays, but I'm hoping for a "cleaner" solution.

Would a mechanical stop in conjunction with an "amp" switch to protect the gearmotor be a good idea? I'm thinking that an older automobile power window system would be similar.

Since I rarely delve into electrical projects at work, any ideas or suppliers for components would be greatly appreciated.

Thanks, ~Eric If you can find a safe place to install an absolute position sensor (one that knows where it is even on power up), then you could add an intelligent control system for the ideal solution (maximum flexibility, self-monitoring, safety). But even then, it might be wise to add safety limit switches near the extreme positions if bad things happen past the ends.

Another trick is to design the mechanical drive system so that it simply reverses direction at the extreme limits. In other words, the chute moves back and forth while the motor just keeps going. **Like windshield wipers.** This way there is no end stop to worry about. All you have to deal with then is getting it to stop in the correct locations, but that may be much less critical and perhaps can better tolerate failures.

Limit "switches" are the best way to go, although I would try to use inductive proximity switches if possible. Even if the chute is not ferrous metal, you can attach a small metal target to it.
An "amp switch" would be expensive and tricky to set up to eliminate false trips. You would also be subjecting the mechanism to stress each time you went to a limit.
I would use (2) prox switches and a micro PLC. I tend to favor Allen Bradley MicroLogix but others are available. They cost well under $200 and considering the wiring you save and the flexibility I think they are worth it.
AB has a free version of the programming software for the small MicroLogix's.
Automation Direct (and others) have small PLCs with inexpensive software.
If you do it with relays, it requires (2) 3 pole relays to reverse a DC motor and detect the limits.
Charlie

Charlie Gill
-- Why not just stop the gear teeth at 180 and 0 and then undercut the rest of the tooth area. That way the worm can run without a limit switch or proximity sensor or anything else unless the operator needs feedback that the limit has been reached. The trick is getting the gear to re-start when the direction is reversed. I have worked with this type of system before with the exception being that the gears were a rod and nut. If you monitor the motor back emf then you should be able to tell when the gear is running free (that would be the fun software part).

Harold
SW SP4.0 OPW SP2 Win XP Pro SP3
Dell 690, Xeon @3.00GHz, 3.25GB RAM
nVidia Quadro FX
Why would it grind OperaHouse? If the gear teeth are undercut to the minor diameter then it should just sit there and go "click, click, click..." until the direction is reversed. Why wouldn't it work on a worm gear as well as it works for a nut and screw? Perhaps I'm missing something but if the gears are aligned right and the pre-load is correct then the gear should just rethread onto the worm. As for being able to tell that the worm is running free that should be detectable by watching the current draw.

Personally I do not like limit switches. Parts wear and the limit switch settings don't remain constant over time. There's always some hysteresis is a limit switch as well. I've had too many of them fail to want them in a system. If you have no choice (software options are not available) then that's fine. When a limit switch fails it doesn't make grinding sounds. The motor dogs down until the gear head strips and then there's a bigger problem.

Just my opinion based on my experiences so feel free to ignore it!

Harold
SW SP4.0 OPW SP2 Win XP Pro SP3
Dell 690, Xeon @3.00GHz, 3.25GB RAM
nVidia Quadro FX
Thanks for all of your helpful replies!!

I guess I was being a little naïve to think I could effectively eliminate limit switches from this project. I would still like to know how older (’s - 90’s) automobile power window systems work. I’ve never seen one use limit switches, and I don’t think the motor is monitored by a control unit. Anybody know what the concept is?

VE1BLL/CharlieGill:
I think you’re both right on as far as the ideal solution (max flexibility, safety, etc), but for now I would like to limit the travel of the chute without utilizing a control system/PLC. Although that may change by the time this project is complete.

OperaHouse:
A DPDT switch, 2 diodes and 2 limit/proximity switches would be fairly simple.

The 12V DC gearmotor has a “starting” current of 12 A. Are there any recommendations for good limit switch suppliers/components that will handle this current? Like CharlieGill suggested, I think an inductive proximity switch (or any sealed magnetic type switch) suitable for agricultural applications would be best.
One concept I've never had the chance to implement is that limit switches (or functional equivalent) should be installed in a location so that if they fail, they're destroyed. This may inspire them not to fail (joke).

More seriously, this sort of approach may provide a simple and inherent fail-safe if you can arrange things so that the resultant mechanical overshoot either opens the circuit, or shorts it to ground (and thus opens a breaker).

For example - imagine a glass fuse mounted at the ultimate mechanical limit of motion.

A simple current limiting method is to use a lamp in series. This does not work if the motor is loaded close to its maximum. Resistance of the lamp greatly increases as current rises. I have used in place of fuses when diagnosing automotive problems. Windshield wipers easily work with a headlight lamp. Recommended that you have very soft stops to allow time for the lamp to heat.

A variation of this is to use resettable PTTC thermal fuses. Specs are like nailing jelly to a tree. Best to greatly undersize the fuse to actual current drawn compared to fuse rating.

The trouble with methods like this are the human operator. Typically bored and easy to amuse, I think they would just hang on a chute and let the teeth make music with the chopped off teeth method. I expect slamming into a stop would be a typical mode of operation for an operator.

I had a bus manufacturer that wanted to eliminate switches on a wheel chair lift with a current sensor. I made him buy a graphing DVM to monitor current. He was thrilled to get an excuise to buy one. Once he got the data, it was doubtful that even a micro could figure out the wild current swings from an end of travel stop. I still think that the ultimate solution is the one I proposed at the very top (1st reply, 2nd para).

Make the drive mechanism inherently back-and-forth with no end stops (just like windshield wipers). Then the only remaining problem is to have the chute auto-park in the correct locations.

With this approach there is no destructive failure mode. If it overshoots the mark, the mechanism just turns around anyway and comes back from the other direction.

And furthermore, an operator over-ride system can be provided in case the auto park sensors fail.