Rack and Pinion Shaft Encoder or Resolver Measuring Systems
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The fundamental operation of Reliance/SEL Series Precision Rack and Pinion Linear Encoder or Resolver Measuring Systems
Precision measuring in the environment of machine tools is a tough job. The most important component in providing the ultimate in both reliability and precision is a rack and pinion linear encoding device.
A Rack and Pinion Linear Encoder or Resolver System incorporates lengths of precision measuring racks; and enclosed channel; and an incremental rotary encoder or resolver with an integral pinion. Our racks are ground with a crown which reduces the need for precision pinion alignment. As motion occurs, the pinion rotates along the racks causing the encoder/resolver contained within the sealed housing to rotate. This rotation reflects the change in linear positions, and the velocity is determined by the frequency of pulses coming from the encoder/resolver.
The channel is a piece of steel, physically connected to the machine. Constructed of machine steel, the channel has the same thermal coefficient of expansion as machinery and will expand and contract at the same rate.
The precision racks are hardened and ground #416 Stainless Steel. The racks are mounted with pitch line on the neutral axis of the channel and laser calibrated to avoid cumulative errors. The rack, mounted to the channel, is surrounded by seals over the entire length of travel to prevent contamination. As an additional precaution against particles adversely affecting performance, the racks teeth are facing down. If any contaminants to penetrate the lipped seals, they will fall harmlessly to the bottom of the housing or are wiped away by the wiping action of the pinion and rack teeth.
The pinion is hobbed to AGMA Q15 integral in the shaft of the rotary encoder/resolver. The pinion is lightly spring loaded against the rack to keep the total backlash under .0001 inches. The pinion is constructed of a metal softer than the ground rack to allow the pinion to be honed to the rack pattern. Once matched to the rack pattern, there will be negligible wear on the pinion due to the light preload and minimal encoder inertia.
Years of Trouble Free Operation
A rack and pinion style incremental linear encoder or resolver combined with a ruggedly constructed DRO, NC, CNC or PLC provides the user with years of trouble-free operation.
Tolerates Coolant and Metal Chips
A rack and pinion style encoder or resolver is the most reliable feedback device for operation in an environment where coolant and metal chips are ever present. One advantage of the SEL Series linear encoder or resolver package is that coolant does not affect either the rack or the pinion. By comparison, a glass scale will either fail or require ongoing cleaning and maintenance after coolant seeps through the encoder seals.
The SEL Series features a unique dual lip seal to inhibit the flow of metal chips into the rack and pinion area. When a metal chip penetrates the single seal on a glass or magnetic scale, it will often render the feedback device inoperable causing a need for system repair and most likely replacement. If a metal chip does enter the SEL Series enclosure, a flexplate simply flexes and allows the pinion to ride over the metal chip without damage to the encoder or error in the readout at final position.
Long Life in Hostile Environment
It is well accepted that the rack and pinion linear systems have a high degree of dependability and life while operating in the rugged environment of the machine tool. We have not had a rack wear out in over 20 years of providing crowned, ground racks for precision position feedback.
Maintenance Free Operation
The SEL Series Rack and Pinion Systems are rarely affected by the environment and tolerate metal chips and coolant. Under normal operating conditions, the rack and pinion system will not require maintenance over the life of the unit.
Operates in Shock and Vibration Conditions
The Linear Encoder or Resolver Package is designed to operate in shock conditions. Specifications far exceed those of glass or magnetic scales. If fact, a glass scale will likely break under conditions approaching the upper limits of a rack and pinion specification.
Accuracy on a machine tool is dependent on the readout systems ability to measure the actual position of the tool relative to the work piece. It is critical that the feedback device adapt to temperature changes without creating errors.
There are two main reasons why a rack and pinion feedback measuring system creates fewer temperature related errors:
A resolution of 50 millionths of an inch (.000050") provides fine resolution for grinding applications and .0001 inch diameter increments for lathes, milling machines or any other machines.
Unlimited Length of Travel
Due to the unique construction of the SEL Series Rack and Pinion Encoder or Resolver Measuring System Package, there is really no limitation to the length of travel. The SEL Series is accurate to +/- .0005 inches regardless of length. There is no cumulated error.
Integral Pinion Transducers
Both resolvers and encoders are totally sealed with MS connectors; and are available with either 1 mm or 1/20 inch circular pitch, 20 tooth integral (quality 15) pinions for 0.002 mm or 0.001 inch resolution as commonly required. 10 tooth, 1/10 inch circular pitch pinion shaft encoders for interfacing with 0.100 inch pitch rack are available also. Other resolutions are available to 0.00002 inches (or 0.0004mm)
Our unique Flexplates eliminate all system backlash and pivot around the rack pitch line permitting as much as 0.015 inch variation between the fixed and moving members without affecting the measuring accuracy. Repeatability in within 0.000025 over the total length of travel.
Neutral Axis Concept
The Rack Channel has been designed such that neutral axes in bending intersect at the pitch line of the rack. This theory has been proven in practice. The calibrated accuracy of the rack assembly is not altered when it is mounted on a machine. This can result in significant savings for machinery builders because mounting surfaces need not be perfectly flat (+/-0.0001 change in accuracy).
This is an non-contact system at the point of converting a mechanical motion into an electronic position signal. In any non-contacting measuring system a precision air gap must be maintained between the moving member and the sensors. Our air gap is within the rotating transducer and accurately maintained by high quality bearings. It is easier to maintain closer tolerances with instrument size components with small ABEC 7 ball bearings than a large machine member with large bearings.
Total measuring accuracies within 0.001 inch over most any length of travel are commonplace with either open rack or protected rack channel; and accuracies within 0.0005 are possible by use of laser calibration of special Grade 4 rack on a machine combined with Quality 15 pinion shaft encoders or resolvers.
With Protected Rack Channel, 0.0001 bi-directional repeatability can be expected.
Angular Measuring Rack Systems
Our AMR (Angular Measuring Rack) system is supplied as a rack set, to suit a specified diameter. The range is from 570 mm upwards with close intervals between possible working diameters. When included in an initial design concept, the AMR gives particular benefits. The measuring radius is a large percentage, usually from 50 to 80 percent of the maximum working radius producing high repeatability.
No axial space is needed below the table. This makes it
lower and stiffer along with other parts of the machine increasing machine working rates
For more information or ordering assistance you may contact us at:
Schlenker Enterprises, Ltd.
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