On the face of it, an off-the-shelf housed linear actuator is always going to be a better buy than a bare ballscrew because all the mounting engineering work is done for you.
Yet demand for ballscrews continues to grow at a steady 5% or more, year on year.
There’s little doubt that ballscrews are one of the best ways of creating linear motion. They convert a motor shaft’s rotation into linear translation, are efficient, robust, controllable and accurate, and they don’t need a compressed air supply or a complex and expensive power pack.
But, considering that they’re often used in arduous industrial environments, they can be a bit delicate. Also, mounting them onto a machine can be more difficult than first imagined.
So why not have the manufacturers build them into a protective cartridge at their factory, so that on site they only have to be bolted into place? After all, the idea works with audio and videotape and many types of fragile filter membrane. Indeed, open up a housed linear actuator and you’re likely to find a ballscrew inside. So the principle is in use, which leaves the question: when should you use an actuator and when do you specify a ballscrew?
Selection depends on the application, and there are many criteria to be considered.The first thing most engineers have to think about is cost. In terms of purchase price a ballscrew is cheaper. However, the following factors also affect the cost:
A) Time taken in calculation of the static and dynamic loads, and specification of the ballscrew
B) Engineering work to mount the ballscrew in place, including turning both ends attaching journal bearings, machining a block to connect the ballscrew nut to the load and accurately fixing guide bearings along the travel range of the nut.
Many companies can work out the cost of the necessary in-house work, and this will usually drive them to conclude that a packaged actuator is cheaper. However, there are plenty of times when it is better to ‘take a hit’ on the internal budgets in order to reduce the cash outlay. It should also be noted that if the application requires only a simple type of ballscrew installation, this could be achieved at such low cost that an actuator is uncompetitive. Likewise, if the drive is to be used on one of a series of identical machines, the in-house costs are spread over the production run.
After price, most people look to value, and there’s a lot of hidden value in an actuator. The engineering calculations and metal cutting are done by the supplier, and this is probably guaranteed for 12 or 24 months under the manufacturer’s warranty.
But this added value is gained at the cost of flexibility. A bare ballscrew is a basic engineering component and as such offers the maximum amount of flexibility. For instance, ballscrews are regularly installed to travel lengths of 5m or more, although specialist expertise is usually needed for these demanding situations. In contrast, actuator manufacturers will be far more circumspect when it comes to extended travel applications.
The environments where ballscrews and actuators are installed, whether industrial or elsewhere are rarely benign, so thought must be given to protection against damage. Ballscrews are quite robust, and may require little protection beyond some common sense and forethought by the system designer. Quite possibly the only protection required will be some simple brushes to clear the screw thread of dirt and debris ahead of the nut’s progression.
If more protection is needed, it may be worth considering an actuator, because the housing will provide a lot of protection. If the housing is not quite enough, then special seals and similar measures can increase the protection further.
But for the most arduous environments, the wise designer will probably go back to the ballscrew, because it can be made with appropriate coatings or even completely from exotic materials. The guides and mountings can be specified similarly. The cost of converting all the components in an actuator to this level of robustness would be prohibitive.
It is a truism that machine builders cannot help but place constraints on their suppliers and machine elements. Perhaps the most common constraint is size and space. Usually, linear drives, whether ballscrews or actuators, are required to fit into a given spatial envelope. When space is particularly tight, a ballscrew will probably triumph over an actuator.
Another issue is if the machine and its drives are destined for use in a remote location that is hard to service, or in an area of the world where skill levels are low. A housed unit is going to be hardier than an exposed one, but it may be more practical to work on the premise that the flexibility of a simple ballscrew will reduce the logistics requirement of repair/replacement significantly.
Both ballscrews and actuators find uses in an increasingly broad range of areas, hence the growth in the market. This growth is attributed to advancing automation. Obviously, machine to machine centres have many linear axes, each requiring a drive. Typically this is the province of the bare ballscrew because of the serial nature of production.
Special purpose machines for assembly, packaging and other industrial processes tend to be built in smaller numbers. But there are many other factors that affect the designer’s choice.
The word automation also covers relatively simple tasks such as lifting/lowering a pallet, opening/closing doors and vents, tilting barrels and drums. This is nearly always a job for a housed actuator.
Beyond the manufacturing industry, ballscrews and actuators have found uses in stage props, museum demonstrations, medical, surgical and dental equipment, to control the canards on guided missiles, to vent fire and smoke in intelligent buildings.
In fact their use is limited only by the imagination, but every time one is specified it is to be hoped that the designer has thought carefully before choosing between actuator and ballscrew.