USA Inc. COST EFFECTIVE HIGH PRECISION LINEAR & ROTARY AXES

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, , 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 , and 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

REDEX HIGH QUALITY PLANETARY REDUCERS Four standard families of planetary T_DRP reducers optimized for stiffness & precision are available. DRP, KRP and KRPX reducers are dedicated to rack & or pinion ring gear drives with radial and torsionally rigid integral output pinions. SRP is a versatile unit that can be integrated with split T_KRPX KRP pinions, ballscrews, or pinions. ZERO- 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 SRP D_DRP product that is easiest to integrate into the new design from a library of standard products.

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MODULAR RACK SYSTEM 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

Standard Range of Racks components that can be manufactured in lots, price and delivery are reduced.

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 .

CROSS ROLLER BEARING SLEWING RINGS, RING GEARS & GEAR SEGMENTS 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. Cross Roller bearing slewing ring with Quantity production once again ensures cost compet- integral external itiveness. 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). Gear Segment sometimes referred to as curved rack

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AUTOMATIC LUBRICATION SYSTEM 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.

ZERO-BACKLASH 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.

Force Diagram – Spring Loaded Force Diagram – Torsionally PDP Split Pinion wound DualDRIVE

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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 preload system that cancels the backlash between 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 PDP Split Pinion 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

SRP Reducer including capacity ranges from 46 – 312 NM with full preload. PDP Split Pinion

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 Redex DRP DualDRIVE 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)

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Electrically Preloaded Systems Electrically preloaded systems use two servo-motors and achieve the preload by braking one pinion against the other. One pinion is considered the driving pinion while the other is considered the braking pinion. These systems are able to change the magnitude and direction of the preload so that the two motors can work together while accelerating large masses.

Force Diagram – Electrically Preloaded TwinDRIVE

Redex TwinDRIVE The Redex TwinDRIVE uses two reducers, two pinions and two servomotors. One servomotor is a master and the second is the slave. The reducers are similar to the DualDRIVE except the mechanical preload is eliminated.

The best choice between mechanical and electrical preload solutions is a function of the control scheme that the machine manufacturer prefers. Mechanically preloaded systems have a single motor controller scheme and electrically preloaded systems require a more complicated two-motor controller scheme. Machine tools use CNC control which already includes the algorithm for controlling two motors. Most Robots use a PLC controller which does not include this algorithm so DualDRIVE systems are more TwinDRIVE Kit consisting of 2x KRP reducers with prevalent on robot seventh and eighth axes. integral output pinions and input flange for servo-motor

APPLICATION EXAMPLES & BENEFITS -Gantry (Portal) Milling Machines Linear axes drives The X-Axis is driven by 2 TwinDRIVE DRP gearboxes, with one on each side of the gantry, combined with hard- ened & ground helical rack. The Y-axis is driven by 2 TwinDRIVE KRPX gearboxes, with one on each side of the spindle column, combined with high-quality helical rack. Finally, the Z-axis is driven by a single SRP reducer coupled to a ground ballscrew. These precise ZERO-BACKLASH drives combined with CNC control ensured a volumetric positioning accuracy of 0.0006” (.015 mm) at the tool point even with an X-Axis greater than 35 feet (10 M) long.

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Furthermore, the stiffness of the gearboxes ensured an acceleration rate of 0.20 g, up to a speed of 1000 in./min. (25 M/min) with very little vibration achieving a good surface finish on the machined part.

COMPOSITE MACHINERY – Automated Fiber Placement

In this application a robot carries a head that unwinds carbon fiber tows and applies them to a rotating mold to make a pressure vessel. An Andantex DualDRIVE KRP kit is combined with high precision rack to drive the linear robot 7th axis with ZERO-BACKLASH. The rotating mold is driven on both ends using ZERO-BACKLASH TwinDRIVE KRP kits combined with an externally geared cross roller bearing slewing ring. Perfect synchronization between the linear and rotary axes is required to avoid bulges on the curved section of the part. Any bulge in the applied material would cause the product to be non-compliant. Since the robot moves back and forth and the mold can be eccentric, there are torque reversals on both axes that would certainly cause bulges in the part if there was any backlash in the system. The Andantex TwinDRIVE and DualDRIVE systems with ZEROBACKLASH ensure that the axes synchronize and the part is compliant. Due to the reducer, pinion and rack precision, position is controlled by an encoder on the servo-motor eliminating the need for expensive linear scales.

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Both axes use an automatic lubrication system to ensure life-long lubrication of linear guides, rack and pinions on the robot 7th axis; gears, pinions and bearings on the rotary axis.

FlexxPUMP 402 DLS lubricant delivery system

Open Cellular Polyurethane Foam Applications for Linear Bearings

Open Cellular Polyurethane Foam Applicator for Rack & Pinions

Robot 7th Axis Rack, Pinions, and Linear Bearings Automatic Lubrication Scheme

CONCLUSION High quality standard components are easily combined together to provide high performance linear and rotary axis drives in many industries including machine-tool, composite machinery, material handling robots, ultrasonic inspection machines and cutting machines to name a few. The standardized design and library of components ensure cost effective solutions that meet machine requirements.

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