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Trends in specifications for industrial linear actuators

Trends in specifications for industrial linear actuators

Manufacturers of industrial linear actuators are seeing increasing demand for higher ingress protection ratings and a growing interest in more advanced controls. So what is driving these trends? We asked the experts at Thomson Industries.

Industrial actuators have operated in harsh environ-ments for years. Recently, the minimum environ-mental requirements have become more rigorous for applications in agriculture, construction, garden, health and fitness, medical and industrial. The IP rating system remains the same: so what is changing and why?

The IP rating specifies the degree of environmental protection an electrical enclosure has against intrusion by foreign bodies and moisture. The first digit indicates the level of protection against solid objects and the second digit indicates the level of protection against liquids. For dusty applications IP5X is usually the minimum rating as it offers limited protection against dust. However, in areas with more substantial detritus, such as woodworking or paper manufacture, IP6X may be suitable as it allows for complete protection against ingress of solid objects. For an outdoor application, IPX6 may be required to withstand heavy rain. However, if an actuator is going to see momentary immersion in water, IPX7 is required.

For applications where liquid ingress is more of a concern, an IPX9k rating may be appropriate. One good example would be the mobile off-highway vehicle industry.  Many large farm and construction equipment manufacturers actually require IPX9k as their equipment is often subjected to high pressure, high temperature wash-downs.  

A common misconception with IP ratings is that a higher IP rating number is automatically better. This is not necessarily the case as a device rated to IP69K does not automatically meet IP67. Equally, IP67 does not necessarily meet IP66. Each rating is designated by unique tests that are not cumulative. For example, an IPX6 test is carried out on a static product where water is sprayed directly on it, but it does not get tested for submersion like a test for IPX7 or IPX8 would. The intensity of the direct spray for liquid ingress tests can range from falling drops of water up to the equivalent of a fire hose directed at the product from specific angles. Tests for IPX7 and IPX8 involve complete immersion in water, which is unlike an IPX6 test that only test for direct spray.

One factor that is driving this enhanced environmental requirement is the fact that more intelligence is also being added locally to actuators and there is greater risk of exposing the electronic components to moisture.

Actuators used to be controlled by changing the current direction to run the actuator in and out. Although simple, this method of control leaves a number of process difficulties such as: what happens if you do not switch the actuator at end of stroke; how do you know the position of the actuator; and the need to accommodate thick cables between the control and the actuator in heavy duty applications. Over the years these problems have been addressed with solutions such as limit switches, overload clutches and overcurrent trip mechanisms to save the actuator at the end of stroke. Positioning can further be controlled using a potentiometer or with encoder feedback for an absolute or calculated position. High current switching is also moved inside the actuator with low level switching outside.

Modern actuators are available with sophisticated control circuitry enabling optimised motion that is verified and safer. The intelligence within the actuator means it can provide features such as load monitoring, mid-stroke overload sensing, soft start and accurate positioning.

These intelligent actuators can be programmed internally to solve tasks, similar to a traditional actuator, or can be controlled over a communications bus by a central control system. A central control system gives great versatility and meets different application needs within the machine or machine range that may be driven by mechanically identical actuators.

Actuators connected to a communications bus provide the means for a control system to monitor position, status, speed and force. Diagnostic and error information is fed back to the control which can identify problems and warn the operator if the actuator needs replacing in the near future. The internal intelligence within the actuator also replaces ancillary items such as limit switches and removes the need for large cables to be routed through the machine.

There is no need for relays and MOSFETs in the external control, no high current spikes, lower power demand, and this results in lower energy used. This makes the installation more compact and gives engineers a greater freedom in design to improve areas such as ergonomics and add richer sets of features. Having an actuator carefully controlled via a communications bus also means that it can be installed further away from the operator. This enables advanced control in harsh environments without exposing people to potentially hazardous conditions.

As markets specify higher protection levels for actuators and require greater intelligence, what challenges does this present to manufacturers? Space is needed within the actuator to house electronics and moisture levels need to be considered with cycling temperatures, requiring a vent and seal system that can withstand necessary IP testing.

Actuators are most vulnerable when they are extended and the integrity of the seal between the housing and the rod must be tested to the most extreme scenario. A wiper cleans as the rod moves back and forth to prevent ingress of dirt into the housing and seals prevent from water ingress. Many specifications also call for wider temperature ranges and this impacts on the choice of materials for the wiper and seal. It is difficult for manufacturers to achieve IP67 or IP69K, thus most products on the market place have a rating of IP65.

As well as tougher IP ratings, many OEMs in areas such as agriculture also require resistance to chemicals such as fertilisers, hydraulic fluids and brake fluids. These can be aggressive to plastics and metals and make the choice of seal material critical.

Manufactures like Thomson Industries offer actuators that meet IP66, IP67 and IP69K to suit such application needs. The Thomson product engineering team use extreme test conditions to verify the robustness of products such as their Max Jac and Electrak throttle actuators that are designed to handle some of the harshest environments. They are tested against the most commonly used chemicals to ensure seal and enclosure integrity. Tests combine vibration, temperature cycling and chemical exposure. Thomson actuators may be customised to exact application needs, including the use of special paints or coatings to meet specific environment specifications.

Actuator manufacturers are challenged to respond to the modern requirements for more intelligent machinery and improved safety. Other trends that Thomson engineers see in combination with intelligent control and higher IP rating include requests for higher load capacities and more compact designs. These greater efficiencies further enable designers to offer more value within their machine design and a differentiated position in the market place.

Thomson actuator engineers have a 40 plus year track record of listening to customers and identifying important trends. Their response to customer needs is a continuous program of R&D focused on providing machine manufacturers with products optimised for modern market needs, without compromise on performance.

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