Screw Compressor Rotors Research Articles

A geometric approach to calculating tool wear in screw rotor machining

Screw compressor rotors are machined today by grinding or milling, usually in two stages; roughing, when the work piece is machined to its approximate size, and finishing, when the rotors are machined to their final dimensions. The material, or cutting stock, which is removed in the finishing operation, is determined by the design of both the cutting tools. Since screw compressor rotors are of a helical shape, each point of the tool traverses a different contact length during the cutting process. Thus the rate of wear of the finishing tool, along its profile is not uniform. The envelope theory of gearing is used here to calculate the relative motion between each point of the tool and the rotor during the cutting process. The rate of tool wear at each point is then calculated on the assumption that it is proportional to this relative motion. The calculation results are compared with experimental values of rates of tool wear and good agreement is noticed. On this basis, it is shown how to produce a roughing tool that results in semi-finished stock of variable thickness that enables the finishing tool to wear at a uniform rate. A similar technique is applicable to many machining processes in which form tools are used.

Profile generation method for twin screw compressor rotors based on the meshing line

A method for generation of the profile of twin screw compressor rotors from a meshing line is analytically derived. The solution obtained depends only on the distance between the rotor axis, the lobe number of both rotors and the given meshing line description. Requirements of the meshing line are listed and an analysis is carried out to obtain optimal profile design. Application of the method is demonstrated.

Analysis of Rotor Contact for Screw Compressor

A reliability evaluation of meshing rotor surfaces needs contact theory on a micro size area. We propose a method to evaluate whether a screw compressor rotor has enough contact fatigue strength. The method is based on analyses of three dimensional curvatures and the Hertz contact pressure. At a contact point one of the principal curvatures directs the rotor's sealing line and this is calculated as the gap distribution. Another perpendicular curvature is calculated with an approximate arc to the rotor profile. As the contact point is an inflection point, the contact condition is first calculated on each side and then composed. Using our method we found the rotors of a typical screw compressor have enough strength. This is because the rotor contact pressure is calculated to be 0.57 times of a contact fatigue strength of their material.

Optimisation of screw compressors

Increasing demands for more efficient screw compressors require that compressor designs are tailored upon their duty, capacity and manufacturing capability. A suitable procedure for optimisation of the screw compressor shape, size, dimension and operating parameters is described here, which results in the most appropriate design for a given compressor application and fluid. It is based on a rack generation algorithm for rotor profile combined with a numerical model of the compressor fluid flow and thermodynamic processes. Some optimisation issues of the rotor profile and compressor parts are discussed, using 5/6 screw compressor rotors to present the results. It is shown that the optimum rotor profile, compressor speed, oil flow rate and temperature may significantly differ when compressing different gases or vapours or if working at the oil-free or oil-flooded mode of operation. Compressors thus designed achieve higher delivery rates and better efficiencies than those using traditional approaches, which is illustrated in an example of the 3/5 screw rotors designed for a family of dry air compressors, produced and marketed by a renown British compressor manufacturer.

Gear-Cutting Tool for Screw-Compressor Rotors

A built-up hob for a screw-compressor rotor is proposed. The hob is composed of cutter blades and a hob head. The blade is cut out from a tempered planar plate of high speed tool steel by wire-cut electrodischarge machining (EDM). During the wire cutting process, the wire electrode has a fixed angle to ensure both the side and the end relief angle of the cutting edge, so that the rake face regrinding sharpens the cutting edge without changing the profile of the rake face. In other words, theoretically, the hob has no accuracy deterioration due to rake face regrinding. The formulae of rack tooth profile are derived from the female rotor tooth profile expressed by numerical formulae. Next, the fundamental helicoid of the hob and the profile of the cutting edge of the blade are calculated. The hob was fabricated and the rotor hobbing was carried out.

Gear-Cutting Tool for Screw-Compressor Rotors. Built-up Fraise.

Gear-Cutting Tool for Screw-Compressor Rotors. Built-up Fraise.

Measurement of Screw-Compressor Rotor Using Coordinate Measuring Machine.

A method for measuring a screw-compressor rotor using a coordinate measuring machine(CMM)is proposed. The profile of the rotor is constituted by a number of profile curves that are smoothly connected with each other. The position of each connecting point can be expressed with its radius in cylindrical coordinates. Such a rotor tooth surface is measured with a CMM. During the measurement, the current curves should be identified and distinguished from the rest. To do this, we compare the radius with the distance between the measured point and the Z-axis in cylindrical coordinates. The essence of the method is as follows. First, the coordinates of a number of points on the surface are measured. Second, the probable rotor surface is estimated by the method of least squares, and the errors are determined. This paper explains in some detail methods for measurement of eccentricity, of errors due to the division of the tooth space, of profile errors, and of lead errors.

Gear-Cutting Tool for Screw-Compressor Rotors. Built-up Hob.

A built-up hob for a screw-compressor rotor is proposed. The hob is composed of cutter blades and a hob head. The blade is cut out from a tempering planar plate of high speed tool steel by wire-cut electric discharge machining. During the wire cutting process, the wire electrode has a fixed angle to ensure both the side and the end relief angle of the cutting edge, so that the rake face regrinding sharpens the cutting edge without changing the profile of the rake face. The formulae of rack tooth profile are derived from the female rotor tooth profile expressed by numerical formulae. Then, the fundamental helicoid of the hob and the profile of the cutting edge of the blade are calculated. The hob was made and the rotor hobbing was carried out.

On gearing of helical screw compressor rotors

Twin-screw compressor rotors are effectively helical gears. When these are formed from a hobbing cutter, the hobbing tool and the rotor together constitute a pair of crossed helical gears. In the present paper, the envelope gearing method is used to derive a meshing condition for crossed helical gears which is then used to create the profile of a hobbing tool. A reverse transformation enables the rotor profile thereby manufactured to be calculated. Simplification of the main gearing condition leads to the meshing expression for helical gears which may be used for the design of screw compressor rotors.

Impact Vibration Analysis of Screw Compressor Rotor System. 3rd Report. Analysis in consideration of Inertia of both rotors.

The oil flooded screw compressor rotates with contact between the corresponding male and female rotors. As gas pressure rises in the grooves of the rotor, a periodically fluctuating torque occurs. An impact vibration between the male and female rotors is induced by the torque. In this report, impact vibration analysis is carried out considering movement of both rotors. The stable region of the periodical impact vibration, which causes a noise of compressor, is calculated. Except this region, the impact damped vibration only occurs, where the compressor is driven quietly.

Impact Vibration Analysis of Screw Compressor Rotor. 2nd Report. In the Case That Impact Vibration Occurs at Both the Driving and Trailing Sides.

Impact Vibration Analysis of Screw Compressor Rotor. 2nd Report. In the Case That Impact Vibration Occurs at Both the Driving and Trailing Sides.

Calculation of Gas Loads Acting on Screw-Compressor Rotors.

A calculation method of rotor loads and some numerical examples are presented. Gas loads acting on the male and female rotors are calculated from sealing lines which are formed as border lines between working spaces. The procedure is simplified by taking advantage of the property that the sealing lines do not change in shape when the meshing rotors rotate. Axial loads as well as radial loads are obtained from sealing line shapes projected to two planes which are parallel to the rotor axis and intersect perpendicularly. Especially, the axial loads are calculated using the principle of virtual work. The calculation results show that a high operating pressure itself does not necessarily increase the amplitude of the rotor load, but the mismatching between the operating pressure ratio and the built-in pressure ratio makes the amplitude larger.

Axial and Torsional Coupled Vibration Analysis of Screw Compressor Rotor.

Axial and Torsional Coupled Vibration Analysis of Screw Compressor Rotor.

Analysis of Static Transmission Error of Screw Compressor Rotors by a Rigid Model.

A numerical method of calculating the static transmission error of screw compressor rotors is presented. Factors such as profile errors, lead errors, bearing position errors, and misalignment of the rotors with respect to the rotational shafts are considered in the calculation. All compressor parts are assumed to be rigid. Clearance between the rotors in the direction of the rotor rotation is calculated from rotor surface displacements, composed of the above individual errors. By tracing the clearance along the theoretical contact line from the suction end to the discharge end, the minimum value is obtained and it is converted to the transmission error. Some numerical examples of the static transmission error, its Fourier-series coefficients, and movement of the rotor contact point are shown for a sample compressor, of which the shaft lines are not parallel to each other and (or) rotor axes are misaligned.

Metal reinforced plastic helical screw compressor rotor : Moody, H. W. and Bulkley, C. T. (Dunham-Busy, Inc, West Hartford, USA) US Pat 3 918 838 (11 November 1975)

Metal reinforced plastic helical screw compressor rotor : Moody, H. W. and Bulkley, C. T. (Dunham-Busy, Inc, West Hartford, USA) US Pat 3 918 838 (11 November 1975)