The Ball Screws Driving More Compact, High-Performance Machines

Compactness Increases Productivity and Makes Life More Convenient

Think of a smartphone. We walk around with a device that incorporates a high-resolution camera, large memory storage capacity, a high-speed communications system, high-speed processor chips, and other components all in the palm of our hand. This is possible thanks to the development of high-performance parts that are the same size as conventional ones or smaller. It’s no exaggeration to say that making things smaller is at the heart of our modern-day conveniences, and increasing compactness is in high demand in the development of machines as well. Making machines more compact in addition to enhancing their performance facilitates further productivity improvements achieved by utilizing the space they free up.

Methods and Challenges in Making Ball Screws More Compact

Machines combine a drive element made of a ball screw and a motor with a guide element that uses an LM Guide. By incorporating multiple axes, each with an LM Guide, a ball screw, and a motor, they are capable of executing complex operations. This makes small parts crucial when a compact machine design is desired.
At the same time, it begs the question of how to deal with ball screws, which are particularly tricky to size down. The first thing that comes to mind is to select a smaller model number of the ball screw you're currently using. Making the shaft diameter smaller will naturally make the nut outer diameter smaller as well. As a result, the center of the ball screw shaft will be lower, and the machine it's used in will be more compact.
But there’s a catch. A designer reading this will likely have realized it by now, but at this point, we have to consider the trade-offs associated with making things more compact. Going down one size of ball screw makes the shaft and nut diameters smaller, but it also means that the balls incorporated into the nut are smaller as well. This reduces the load that the ball screw can bear and may mean that the service life of the smaller ball screw is shorter than that of the one you’re currently using.
A smaller shaft diameter also affects high-speed performance. This is because, in some cases, a lower critical speed can render high-speed feeding operations impossible. The critical speed of the screw shaft indicates the maximum speed at which it can be used without incurring resonance caused by the screw shaft’s natural frequency. Resonance causes the screw shaft to wobble in a way that can be likened to “galloping” power lines, and this in turn can damage the screw shafts, nuts, and bearings alike. The longer and narrower the screw shaft, the easier it is for this “galloping” to occur.

Both service life and high-speed performance must be considered when making ball screws more compact.

THK’s Solution to the Compactness Trade-Off

But is this trade-off between compactness and both service life and high-speed performance inevitable? THK’s response to this question comes in the form of the Finished End Series Model SDA-VZ, a product that combines all three qualities, provides enhanced service life and high-speed performance, and helps reduce design time.
The primary feature of the Model SDA-VZ is the compact outer diameter of the nut. One of the factors that can impact how big the nut has to be is the way balls recirculate. The most common ball recirculation method involves balls recirculating through a return pipe. The return pipe and components that hold it in place necessitate a nut with a large external diameter. In contrast, the outer diameter of the nut used for the Model SDA-VZ has been made more compact compared to its screw shaft diameter thanks to an ideal ball circulation structure and newly developed circulation part made possible by high-precision machining technology. This has made it possible to use a nut on the Model SDA-VZ that is 30% more compact than one used on a return pipe-style product with the same shaft diameter.
In addition, THK contributes even more directly to making machines more compact by offering a dedicated support unit for low shaft centers to draw out the full potential of this more compact nut.

Achieving Higher Speeds

A new circulation part and an ideal ball circulation structure enable the Model SDA-VZ to achieve a maximum rotational speed of 5,000 min-1 (DN value: 100,000), which is 43% faster than existing models. Here, the DN value is the maximum value at which the product can be used without damaging the circulation part. A ball screw selected with a shaft diameter of ⌀20 and a lead 20 mm long can reach a maximum speed of 96.2 m/min.

Extending Service Life

Two key factors in extending service life are reducing the load applied to the ball screw and increasing the ball screw’s load rating.
The mass transferred by the ball screw plays a large role in reducing the load applied to it. Looking at XY stages capable of a 500 mm stroke in each direction, like the ones pictured below, a stage that uses the Model SDA2020VCZ-3 has a 10% smaller load applied to its lower axis than one that uses an existing model of ball screw (with a shaft diameter of ⌀20 and a lead 20 mm long). Using the compact ball screw Model SDA-VZ with a support unit for low shaft centers allows for a smaller, lighter plate and a lighter load on the ball screw overall.
Given a stroke of 500 mm and an acceleration of 1.0 G, the Model SDA-VZ has a service life 2.2 times longer than an existing model of ball screw.

Enhancing Machine Performance with More Compact Ball Screws

With more compact ball screws, it’s possible to select a model size larger than the ball screws currently used on your existing equipment and thereby enhance the performance of that equipment.

Higher Critical Speeds for Shorter Cycle Times

The critical speed of an existing model with a shaft diameter of ⌀20 and a lead of 20 mm operated over a stroke of 1,000 mm at an acceleration of 1.0 G is 43.6 m/min. In contrast, using a Model SDA-VZ one model size up, with a shaft diameter of ⌀25 and a lead of 20 mm, increases the critical speed by 26% for 55.1 m/min while keeping the external diameter of the nut the same. This reduces the cycle time by about 0.54 seconds.
Enabling high-speed feeding in this way reduces cycle times and ultimately increases productivity.

Increasing the Load Rating

Ball diameter and ball count are key for increasing the load rating of a ball screw. Selecting a Model SDA-VZ with a shaft diameter of ⌀25 and a 20 mm lead, one size up from an existing model of ball screw with a shaft diameter of ⌀20 and a 20 mm lead, increases the load rating by about 73% without changing the external dimensions of equipment. Assuming a stroke of 500 mm and an acceleration of 1.0 G, the Model SDA-VZ that is one size larger also has a service life 3.9 times longer than that of the existing model.

Significantly Reducing Design Time of Machine Drive Elements: A Complete Solution Designed for the Production Floor

Adopting a ball screw drive system involves much more than just selecting a ball screw model number. There are numerous design factors to be considered, like the selection of a support unit and the design of the shaft end shape for securing the threaded shaft and the bracket for securing the ball screw nut. These considerations require time and labor. The Model SDA-VZ has standardized screw shaft end shapes, and THK also offers a support unit that can secure the screw shaft. This makes it easier to design the ball screw installation area and makes it possible to substantially reduce design time. THK continues to provide solutions to customer needs, including but not limited to the demand for increasing compactness.

This article examined how ball screws are being made more compact and THK's related solutions. Technologically breaking free of the trade-offs that plague conventional ball screws in the process of making them more compact by adopting the Model SDA-VZ produces tremendous benefits for equipment and contributes to increased productivity. Please don’t hesitate to contact THK with any concerns you have about making equipment faster and more compact.