Lawn Mower Durability Test Rig- Can you SMASH my Lawn Mower?

It is not often that we are asked deliberately to try and break something, but Cambridge Dynamics staff will rise to any challenge! Following hard on the success of their Lawn Trimmer Production Line (Case Study 2) the same customer invited us to design and build an accelerated life testing rig for rotary lawn mowers that would subject them to all manner of abuse.

Traditionally, mowers have been tested by taking them outside and cutting large areas of grass. Although this obviously is a representative test it involves many hours of work that may extend over more than one grass-cutting season. Our customer identified a requirement for a rig that would subject the mowers to all the conditions they might encounter in typical use, but at an accelerated rate and in a well defined set of tests that could be easily defined and monitored.

The project was handled in two phases. The first phase consisted of fitting a number of typical models of mower with accelerometers and linear potentiometers to measure the shock loads and resultant distortions of the structure that occurred when the mowers were used and abused as might happen in real life. A wealth of data was gathered while the mowers cut grass, twigs, molehills, gravel, large stones and paving slabs. Further data came when the machines were pushed into trees and kerbstones, and bounced down concrete steps. The most dramatic results came from the infamous drop tests. Each mower is expected to survive a number of drops in free air onto concrete from heights up to one metre. Remarkably, the only component on these mostly plastic machines to suffer damage was the metal handle.

Having gathered the data, the second phase of the project was to design and build a rig to replicate the stresses caused by the actions described above. The customer required the rig to test two mowers at a time, and to be capable of running unattended for long periods. Because of the danger from rotating blades and possible flying debris during some of the cutting tests the entire rig was located in an enclosed area with shot-bolt locking of the only access door incorporated into the control system. A web-cam allows the operation of the rigs to be viewed remotely over the customer’s intranet. For each mower the rig can perform any or all of the following actions in any sequence:

  • Hoist the mower to a set height and drop it to the floor.
  • Lift the mower by the handle (to simulate hanging it up for storage).
  • Hold the mower on a rolling road of wooden slats (to simulate pushing it over rough ground). The mower might be running for this test.
  • Hold the mower handle while the rolling road skews from side to side under the mower (to simulate swinging it round on rough ground).
  • Hold the mower handle and lower the rolling road under one side of the mower (to simulate the mower overhanging one edge of a lawn or path).
  • Automatically couple the blade to a dynamometer to allow the motor to run against a variable load.
  • Feed broomsticks, metal rods or sods of earth up into the running blade.
  • Spray the mower with water.
  • Push the mower hard into a fixed stop (to simulate colliding with a kerb-stone).

In order to make the rig as flexible as possible an entire series of tests can be programmed and stored within the control system, which is based on a Telemecanique-Modicon Premium PLC. The operator interacts with the control system by means of a large PC-based colour touch-screen from the Magelis range. The control programme can be defined as a series of actions placed in any order and lasting for any length of time. Nested repeat points mean that programmes that will run for many days can be created in just a few minutes.

The rig has notched up many hours of running time, and has proven to be an effective development tool. In addition to speeding up the life tests it has also proved useful in allowing an identical series of tests to be performed on a given type of mower when any design changes are implemented. This enables the design team to be confident that there are unlikely to be any component failures in the field as a result of the changes to the design.

  • Cambridge Dynamics have also produced test rigs for:
  • Measuring the dynamic force profile of an explosive actuator for unfolding the wings on a cruise missile.
  • Checking the correct operation of a servo-controlled pair of rolls in a steel bar rolling mill.
  • Verifying the strength of ultrasonically welded clips on a power steering component.
  • Confirming the correct crimping of a hydraulic hose end fitting.
  • Measuring the torque required to move a recirculating ball nut along the length of a mating leadscrew.

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