Speed switches – Comparison of Murphy, Hewitt and Bolec

Speed Switches - Comparison of Murphy, Hewitt and Bolec

In this article, we will talk about speed switches, or rev switches.

What is a speed switch?

A speed switch is a device that monitors the frequency of an input signal and does something when the frequency rises above or falls below a pre-determined set point.  The term "speed switch" is technically a misnomer.  Speed switches actually measure frequency which is expressed in Hertz (Hz), or cycles per second.

What are the applications of speed switches?

Speed switches have many applications:

• Engine speed over-speed alarm or shutdown
• Speed sensitive pull-in/drop-out
• Engage or disengage PTO's
• Select between two pressure switch set points according to speed of the engine or transmission and thus give maximum protection of equipment while at operating RPM and eliminate false alarms when at idle
• Machinery speed over-speed alarm or under-speed alarm
• Buses to make doors close when reach a specified speed
• Garbage trucks to disable arms when in motion
• Disable gear change when forklift is in motion
• Disengage feeder to wood chipper when engine speed falls below a pre-set level
• Flashing alarm and / or shutdown when forklift is driven overspeed in factory

What are the benefits of Speed Switches?

Speed switches provide valuable benefits in safety and cost.    When installed into machinery, the speed switch will increase the life of your capital investment because maintenance can be performed proactively and ahead of unexpected failures.  This will reduce downtime, maintenance cost, and lower your material waste cost.   Speed switches will provide better accuracy of your process by keeping your machine speeds within their acceptable operating ranges.

Murphy SS300 and Hewitt Industries 030-300

The Murphy 24V SS300-24 is a single set point speed switch with SPDT relay dry contact output. The trip point is set by a potentiometer. An LED indicates when the signal source is present. A second LED turns on when the trip point is reached. An optional time delay is on board to delay operation of the relay for 2-6 seconds after the set point has been reached. The SS300-24 is intended for installation within a weatherproof enclosure to protect it from rain, dust, etc.

The Hewitt 030-300 is similarly a single channel speed switch with SPDT relay dry contact output.  

For both the Murphy and Hewitt speed switches, the set point is adjusted with a multi-turn potentiometer.

Bolec SS501

The Bolec SS501 is a dual channel speed switch.   Because it is dual channel, it is like getting two of the Murphy or Hewitt single-channel speed switches for the price of one.  But that is not the most brilliant feature of the Bolec SS501 speed switch.

Amazing feature - Built-in frequency counter

The Bolec SS501 has a 4 digit 7-segment display.  When you press the "Display" button, you can see in real time the input frequency.   No need for a frequency counter or oscilloscope in the field.   No more trial and error.   No need to fiddle around with 25-turn potentiometers.   For each channel, you can set two set points.  This creates "hysteresis" so that there is not chattering when the speed is close to the set point.  Our customers love this design because it is so easy and accurate to setup in the field.

Amazing feature #2 - Super easy to set in the field

Another brilliant feature of the Bolec SS501 is that you can program the trip speeds like a digital alarm clock.  Using the onboard 4 digit 7-segment display and three pushbuttons, you can set the trip-point to +/- 1 Hz accuracy.   Try doing that with a device with a 25-turn potentiometer!

Many input and output possibilities

With two separate signal inputs switching two separate output relays there can be a number of combinations for monitoring:

• Two input frequencies giving two separate adjustable outputs
• Two separate input frequencies giving one output
• One input frequency giving two separate outputs
• One input frequency giving one output

12V and 24V

The Bolec SS501 can be both used on 12V and 24V systems.   Just flip a switch on the PCB and you are good to go.  No need to have stock of different units for different voltages.

Loved in Australia.   Now available everywhere.

The Bolec SS501 has been used on many OE and aftermarket applications in Australia.   We can ship internationally within 24 hours.