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8/14/2017 | Q&A with Thomson - Lifts and hoists: Vertical linear motion designs reach new heights

What are some technologies to increase the efficiency of vertical applications?

Gilmer: Typically, a vertical lift must resolve two components of force — lifting force and side-load (or moment load). Technologies to handle those loads range from simple pulley or winch-type lifts and hoists (which only handle the vertical load) to more complex and complete lifting columns that handle both lifting force and side-loads. More complete technologies make for more efficient lifting action. Further integration of electronics (such as feedback sensors and controls) into older-generation lifting devices allows for turnkey setup for controlling the lift in a vertical application.

What are the unique challenges in terms of forces and acceleration on these vertical lifts?

Gilmer: One must consider both the load of what is being lifted vertically and any side or moment loads that the application encounters along the lift. Consider an ergonomic adjustable desk. The application calls for a vertical lift. One challenge here is whether or not the load on the desk is evenly distributed. Lifting from the center may provide excessive moment loading and lead to failure, while lifting from both sides (with two devices) presents offset loading. The latter can in turn cause uneven lifts.

Secondly, there may be forces acting on the desk surface after or during the lifting process that introduce more uneven loading and moment loads for the lifting devices to resolve. This is why one typically sees bearing-supported actuation doing this type of lift. Actuators inside the linear-bearing structure handle the vertical load, while the linear bearing supports keep the structure level and stiff in the presence of uneven loading or side-loads. Keeping multiple devices in sync (given various accelerations and forces due to offset loading) is likely the most common problem in this application type.

What kinds of sensing feedback is common in such setups?

Gilmer: The most common design uses an encoder that communicates position feedback as a pulse-train signal relative to the home or starting position of the lifting device. One caveat here: It’s important to zero out the encoders before a lift is enacted, or else there may be uneven lifting or inaccurate positional feedback.

What are common features to ensure safety in vertical setups?

Gilmer: A common safety measure is another integrated control called either electronic load sensing or current sense. With this, an onboard controller monitors current draw on the lifting device and stops movement if an overload is encountered — or if an end-of-travel limit is reached. Stopping motion in these scenarios ensures the device is operating only when told or when it should move given the application parameters. The load-sensing devices can be calibrated for alternate applications — for example, to allow more current in one direction (lifting) and less in another (lowering) to prevent any sort of pinching or damage to anything that may get caught underneath what’s being lifted. Once the movement is done, designs can incorporate static holding brakes to prevent the load from moving — even when power is lost. Oftentimes these brake designs allow for a larger load than the lift can handle (referred to as static loading capability), which gives the application an appreciable margin of safety.

What's behind the increased use of electric actuators in vertical lift applications?

Gilmer: Electric actuators typically resolve the vertical and lifting forces in these applications. They’re responsible for moving and ultimately holding loads in place after moves are done. The aforementioned controls and electronics are now integrated with electric linear actuator designs to ensure efficient designs. Once the lifting forces are resolved, all that must be done is supporting the actuators with bearing surfaces to prevent side-loading, which will typically damage an electric actuator.

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