Cartridge and block spindles - Gilman USA

Scanning the horizon for enhanced spindle performancePreviously published in Modern Machine Shop June 2002There are a lot of things that a job shop can do to improve theperformance of a machining tool spindle. From predicting spindlereplacement to taking advantage of new high-speed tooling, spindleenhancements can increase uptime and improve performance.The following suggestions come from Chris Hetzer, Vice Presidentof Engineering, Gilman USA (formerly Russell T. Gilman, Inc.).Monitor that spindleInstalling a spindle monitoring and recording unit (SMRU) helpskeep track of critical maintenance issues. Maintenance of spindlesis hard to predict because they are designed to work in a certaintemperature range for a certain period in time. For example,when operated at 15°C above normal, grease-life decreases bya factor of two. SMRUs record temperature, speed and runninghours. In one brand of these units, the Gilman USA iSpindleSMRU, the information is stored in a histogram form which providesthe maintenance engineer with information to assess howlong the spindle has been running and to make a prediction of theremaining grease-life. The unit can be integrated into the spindleor bolted to it. A maximum of 255 uniquely addressable unitscan be connected to a wireless wide area network. PC software isavailable for configuration of the SMRU’s identification, recordingsettings, user comments, real-time display of temperatures andspeed information and database storage in MS Word or HTML.Ceramics are coolConsider replacing a spindle’s bearings with hybrid ceramic bearings.On a grinder, 25 percent higher RPM can be achieved byreplacing steel ball bearings using oil/air-mist running at 10,000to 15,000 RPM with hybrid ceramic bearings and synthetic greaselubricant. Grinding spindles with hybrid ceramics spindles mayrun 4,000 hours without problems, compared to 3,000 hourswith steel bearings. Gilman USA reports running hybrid bearingsup to 5,000 hours at speeds exceeding 2.0 million ndm beforepart degradation occurs. (Ndm is the bearing pitch diameter inmillimeters multiplied by RPM, a value that compares the relativeperformance of bearings of different sizes.) Some steel bearingsquit after 700 hours. Hybrid ceramic bearings can lower temperaturesnearly 50 percent and last as many as seven times longerthan steel bearings. In a horizontal ram-type machining center,switching from steel to hybrid bearings has been shown to dropbearing temperature from 60°C to 36°C at 12,000 RPM.Belt drives can be hip, tooConventional belt-driven spindles, which use a pair of angularcontact bearings at the belt-drive end, may have problems withspindle deflection and premature failures. At high speeds, the beltpull loads the bearings radially and also restricts the axial movementof the bearings to compensate for the thermal growthof the spindle system. In a typical hybrid ceramic roller bearingupgrade for a 40-taper vertical machining spindle operating at15,000 rpm, the rear bearing set is a single row, hybrid ceramicroller bearing with a composite plastic cage, while the front bearingsmay be triplex angular contact hybrid ceramic ball bearingssuch as Gilman USA 71914ACD/P4ATBA. Lubrication is syntheticgrease. The hybrid ceramic roller bearing offers twice the radial rigiditythan the original angular contact set and allows for smoothshaft thermal elongation. The entire spindle system is more rigid,which allows for a higher first flexural critical speed. The operatingperformance of the cylindrical roller bearing with grease may beas high as 1,162,500 ndm at an operating temperature of 36°C.Shielded is fasterUpgrading to shielded ball bearings can eliminate the need for airpurge or spot oil lubrication while reducing contamination and allowingincreased spindle speed. Standard shielded bearings havethe same internal geometry, ball diameter, speed rating, load ratings,preload classes, matching criteria and contact angles as thebearings they replace. Double-shielded bearings are dependentsolely upon the pre-packed lubricant of the bearing. An GilmanUSA high- speed double-shielded design incorporates smaller diameterrolling elements, the rings have been designed with openconformities for high-speed performance. This design has higherspeed capability but lower load ratings than the correspondingsize of the CD/ACD bearing. These high speed shielded greasebearings will run faster than spot oil lubricated standard bearings.Let the “Force” seal itGilman USA’s air seals use an air distribution ring and a rotatingmetal cased elastic sealing member combining the advantages ofcontact seals and air purge. The elastic sealing member slightlylifts from the contact surface as the spindle rotation increasesthe centrifugal force. The metal case slings away any oncomingcontaminants. The air-distribution ring allows for uniform airflow,which ensures that the seal will ride on a thin film of air whilerotating. The air also pushes contaminants away from the spindle.In a static condition the elastic seal provides a positive contact bykeeping contaminants from entering the spindle. The result is ahighly effective seal.HSK means high speed “Kapacity”The German translation of HSK literally means “hollow tapershank,” but upgrading the spindle to HSK tooling will improve rigidityand high-speed machining performance. HSK tooling allowsdeeper cuts and higher feed rates in milling and boring operations.HSK has metal-to-metal contact both radially and axially.The radial stiffness of HSK tool holders is up to five times greaterthan CAT, SK or BT. In the axial direction, the accuracy of a CAT(SK, BT) connection can vary as much as 0.004 inches comparedto an HSK shank. HSK is not subject to bell-mouthing of themachine spindle, which occurs as a result of wear. It also has ahigher natural frequency allowing higher speeds before chatter.HSK tooling transfers significantly greater torque and is moreresistant to pullout forces. The tool-changing capability of HSK isfaster than steep-taper shanks because of the short length of theHSK taper and the lighter weight of its hollow shank. The HSKconnection prevents the shank from pulling back into the receiverduring high-speed operation, changing theZ-axis position.66 ©2011 Gilman USA, LLC