Research into cycloidal gearing gave Olly Dmitriev, CEO of Vert Rotors UK, the idea that the principles of this technology could be applied to a compressor. The new type of oil-free screw compressor developed and patented by Vert Rotors has proved to be 35 % more energy efficient than dry oil-free machines. The main reason for this energy saving is that there is no stator in the design, which is common in rotary twin shaft compressors and which allows the gas being compressed to leak back.
The Vert Rotors compressors have only a pair of male and female screws with virtually no clearance between them, which is where the need for high accuracy 5-axis machining originated for the company. Olly Dmitriev says, “Until recently all micro compressors capable of producing pressures over 2 -3 bar were piston compressors, which produce a lot of vibration. Our rotary design is virtually vibration free, which is vital where the compressor is located next to delicate sensors. Example applications where there is a need for a vibration free compressor include satellites or medical devices. Our new 100W desktop compressor “VERT.04” produces pressure of 8 bar without vibration, and uses small, high accuracy rotors which need to be machined in 5-axis. We looked at a variety of machines and chose the HSC 20 linear . The machine has linear drives and scales unaffected by backlash and the composite granite frame gives exceptional temperature stability making it possible to achieve 5 μm accuracy and very high quality surface finish of Ra 0.15 µm.”
Other factors important in the choice of the HSC 20 linear included the minimal headroom and floor preparation required for the installation of the machine. Some of the other machines considered required the installation of extensive foundations under the machine to ensure accuracy. The DMG MORI machine has a footprint of 3.5 m2, linear drives in X, Y and Z with 2 g acceleration, a shaft cooled spindle for rapid warm-up and thermally stable machining, and a thermosymmetrical gantry design with swivelling rotary table.
Olly Dmitriev continues, “On a good day, with a ball screw machine, we can only achieve 10 μm accuracy due to backlash problems in 5-axis every time we change the cutting direction. Additionally, we get spindle movement due to temperature fluctuation. With linear drives and temperature control these problems are now in the past. The repeatability and accuracy we can achieve with the HSC 20 linear is crucial to our success.”
DMG MORI and Sauer collaborated with Vert Rotors in the development of the machining strategies, reducing machining times from 4 hours to 1 hour. The machining of the helical surfaces is very challenging and uses 1 mm and 3 mm diameter tools and custom lollipop cutters operating at 30,000 rpm.
Vert Rotors has only had the HSC 20 linear since the end of December 2016 and is continually fine tuning the machining process. Olly Dmitriev says, “We are getting CMM reports which show consistency between each side of the rotors and we are building our expertise on the machine all the time. We were confident with the machine from day one.”
For Vert Rotors, the ability to reliably manufacture the small rotors in its VERT.04 desktop compressor comes at an exciting time for the company as there is no equivalent technology on the market. “Our single stage compressor will produce 10 bar with a high flow rate, low power consumption and small size. In addition, as we use water lubrication, the air produced is oil free and at 400 C.” International interest in the revolutionary design is coming from major companies in the aerospace, medical, defence and automotive sectors. Vert Rotors is looking at numerous applications including the use of one of its compressors on a satellite.
Steve Finn, Managing Director of DMG MORI UK says, “This is the first HSC 20 linear to be installed in the UK and we are delighted to be involved in the development of Vert Rotors’ green compressor technology. We are working in partnership with them to develop local expertise in the UK for this level of high precision machining.” Olly Dmitriev adds, “We are keen to build our knowledge and share our skill on this machine with other companies as it can only be good for UK industry. We have made some huge steps forward in just a few weeks which demonstrates the potential of the HSC 20 linear.”