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Andantex USA is a leader in supplying quality motion control components, systems and solutions for industrial machine applications. Andantex USA offers a wide range of high quality servo driven worm and planetary reducers, rack&pinions, lubrication components and systems, Two Speed Spindle Drive Gearboxes, curved racks, gear segments, slewing rings, ring gears and lifting sy

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Cost Effective High Precision Linear & Rotary Axes

POSTED 03/15/2022

 | By: Rack and Pinion Systems

By David Regiec, PE

The performance of today’s high precision servo-driven, computer-controlled machines is directly related to the stiffness and accuracy of their linear & rotary axes drive systems. In order to achieve precise positioning and repeatability of these axes, the mechanical components must be manufactured to very tight tolerances (on the order of 20 microns (0.0008”)). Manufacturing these parts in small quantities (per machine basis) is very expensive and threatens the competitive position of the machine manufacturer. However, component manufacturers who supply high precision mechanical components to many machine manufacturers are able to develop standard ranges of high-quality products that are produced in quantity with well-defined processes at extremely competitive prices. Andantex, USA, a member of the Redex-Group, is able to offer modular system building blocks consisting of Racks, Pinions, Planetary Reducers, and lubrication systems to build linear axes of any length and move weights from 100 to 200,000 Lbs (50 to 91,000 Kg.). Furthermore, standard slewing ring bearings, ring gears, and gear segments are combined with planetary reducers to drive rotary axes from 1.5 ft. (0.50 M) to 100 ft. (30 Meter) in diameter. The mechanical building blocks for a precision axis drive consist of:
• High Quality Planetary Reducers including Output pinions
• Hardened & Ground Racks for Linear Axes
• Cross Roller Bearing Slewing Rings & Ring Gears or Gear Segments for Rotary Axes
• Automatic Lubrication Systems

Four standard families of planetary reducers optimized for stiffness & precision are available. DRP, KRP and KRPX reducers are dedicated to rack & pinion or pinion ring gear drives with radial and torsionally rigid integral output pinions. SRP is a versatile unit that can be integrated with split pinions, ballscrews, couplings or pinions. ZERO-BACKLASH is achieved using DualDRIVE (mechanical preload) or TwinDRIVE (electrical preload). Over 3000 configurations possible offering the perfect balance of performance & price. The machine designer is able to select the product that is easiest to integrate into the new design from a library of standard products.

In order to provide a cost competitive rack & pinion drive system, an industrialized approach to the manufacture of racks had to be developed. This is achieved by offering a range of standard racks with Modules, M, from 2 to 10 (Diametral Pitch = 25.4/M) in lengths of 500 mm, 1000 mm and 2000 mm with straight (spur) or helical teeth in standard quality levels up to DIN 5h22 (~AGMA 2000-A88 Q13). These racks are designed to be linked together (using companion racks to ensure proper tooth spacing) to form an axis of the required length. Since the rack sections are standard components that can be manufactured in lots, price and delivery are reduced.

Standard Range of Racks
The quality and precision of the rack is also critical to the performance of the system. The standard hardened and ground quality AGMA 13 rack has a cumulative pitch error of 0.018 mm/300mm (0.00071 in/Ft.) which is equivalent to a precision ground ball screw.

A standard range of preloaded cross roller bearing slewing rings with bearing race diameters from 1.65 Ft. (0.5 meters) to 10 Ft. (3 meters) provide support for rotary axes.  Bearings can be equipped with integral internal or external ring gears. Gear segments (sometimes referred to as curved racks) are made to order on a 5-Axis milling machine. The standardized range of cross roller bearing slewing rings provide high accuracy, high stiffness and uniform rotational resistance independent of load. Quantity production once again ensures cost competitiveness. Since there is no way to standardize gear segments, the manufacturing method chosen (5-axis milling) provides the most cost-effective manufacturing in small quantity because only conventional milling tools are required as opposed to purchasing gear hobs for each module (or diametral pitch).

A compact automatic relubrication unit is provided in order to ensure life-long lubrication of racks & pinions, open gearing, linear guideways, ballscrews, and bearings. The heart of the system is FlexxPUMP grease delivery unit. These pumps deliver grease to the items being lubricated through industrial strength piston pumps with programmable frequency and delivery rates. They are combined with tubing and open cellular polyurethane foam applicators for open gearing drives. All components are standard, produced in quantity making an industrial strength, cost-effective lubrication system.
Even though rack pitch error is very small, the rack & pinion drive system will not provide the best positioning precision unless the backlash between the rack & pinion is eliminated. In order to eliminate this backlash, two classes of preloaded systems have been developed. The first class is mechanically preloaded systems including the PDP (from WMH-Herion) Split Pinion and the Redex DualDRIVE. The second class is electrically preloaded systems including the Redex TwinDRIVE system.

Mechanically Preloaded Systems
Mechanically preloaded systems achieve the preload by spring loading or torsionally winding one pinion against the other while they are in contact with the rack. One pinion is considered fixed and the other spring loaded.  These systems are driven by only one motor and since they are used in a mechanical closed loop design, the motor size does not have to be increased to accommodate the preload torque. In fact, the motor is sized to move the load and overcome normal system efficiencies.

PDP Split Pinion SRP Reducer including PDP Split Pinion
The PDP is made of two ground WMH-Herion pinions, each ½ the width of the rack, connected together through a patented Hirth coupling preload system that cancels the backlash between rack and pinion providing an accurate positioning axis using one reducer at a competitive price. The preload torque of the PDP is easily adjusted and the units are equipped with an ISO 9409-1 flange for adapting to speed reducers. The PDP can be combined with precision reducers to provide system backlash < 1 arc-minute thereby eliminating the need for two electrically preloaded reducers (with 2 servo motors) in positioning applications. The PDP is offered with straight (spur) or helical gear teeth in modules from 2 to 5 standard (larger modules on request) and accepts speeds up to 1200 RPM. Torque capacity ranges from 46 – 312 NM with full preload.

Redex DualDRIVE
The Redex DualDRIVE system uses two reducers with two output pinions in a closed mechanical loop. A patented, user friendly torsional preload allows easy setting of the preload torque while the pinions are meshing with the rack. The DualDRIVE design has been optimized for use in rack & pinion and pinion ring gear drive systems. The output pinions are integral with the output shafts to optimize gearbox stiffness. The pinion diameters have been selected to ensure that the resonant frequency is high and the torque seen by the gearbox is as small as possible. A universal housing makes integration into the machine easy because the gearbox can be mounted on any surface.

The DualDRIVE is combined with one servo-motor to provide high precision positioning and contouring accuracy on machines with axis weights up to 200,000 Lbs (91,000 Kg.) and rack forces up to 36,000 Lbs. (160,500 N)