Thomsn BSAhSoL43 2/1/8567 Engineering …

1 Engineering Guidelines for Ball and Lead ScrewsCritical Speed Limits Chart for Lead Screws and Rolled Ball Screws4322/21/211/4111/43/85/21/167/83/1 65/413,0002,0001,5001,000900800700600500 400300200100 ROTATIONAL SPEED (RPM)Fixed-FreeSimple-SimpleFixed-Simple Fixed-FixedMAXIMUM LENGTH (INCHES) BETWEEN BEARINGSI nchesmmInchesmmInchesmmInchesmm615210254 1230415381123042050824609307621845730762 3691445114324609401016481219601524541371 9022861092768134340360152410025401213073 1493784661676110279413333781644165721828 1203048145368317945467819811303302158401 3194492784213314035561704318209530890228 6150381018246222245689962438160406419449 2723960701022590170431820652322546451108 2743180457221855372696832120304819048262 3058422847213126320020050802426146298756 9 ABCD307625012706115497519054812198020329 7246311930224210567017788521591052667369 14601524731854902286 Every screw shaft has a rotational speed limit.

2 That is the point at which the rotational speed sets up excessive vibration. This critical point is modified by the type of end bearing support use this chart, determine the required rpm and the maximum length between bearing supports. Next, select one of the four types of end support shown below. The critical speed limit can be found by locating the point at which rpm (horizontal lines) intersects with the unsupported screw length (vertical lines) as modified by the type of supports selected below. We recommend operating at no more than 80% of the critical speed limit to allow for misalignment and/or lack of screw straightness.

3 If speed falls into dotted line, consult : Curves for the screw diameters shown are based on the smallest root (minor) diameter of the standard screws within the nominal size range and truncated at the maximum ball nut rotational speed. DO NOT EXCEED this rpm regardless of screw Motion. life RelationshipColumn Loading Capacities L = life in inches D = Design load Fr = Dynamic load Rating fW = Nominal Operation Operation with impact or vibration For Ball Screws Ball screws are rated for 1,000,000 inches of travel at the rated dynamic load . This is the load at which 90% of a group of identical ball screws will run without flaking for their lifetime.

4 However, they will travel farther than this at lower limits. These load - life relationships are analogous to the B10 rating common in the ball bearing industry. The relationship of load to life is an inverse cube relation. For example, reducing the load by 1/2 increases life eight times. Doubling the load decreases life by 1/8. Every attempt should be made to design for loads that do not exceed the dynamic load rating of the nut. Never exceed twice the rated dynamic load rating of the nut while in motion. To use the load / life equation, look up the rated dynamic load for the assembly you are interested in. Use a diagram load that covers your typical worst case loading and compute the predicted theoretical design life as follows: L = ( )3 x 1 x 106 FrD fw BSA assumes no liability for assemblies used at above the dynamic load rating of the nut.

5 85 Lead and Ball Screws477,000 COMPRESSION load (LBS)254,000138,000100,00090,00080,00070 ,00060,00050,00040,00030,00020,00015,000 10,0009,0008,0007,0006,0005,0004,0003,00 02,0001,5001,000900800700600500400300200 100 MAXIMUM LENGTH (INCHES) BETWEEN BEARINGS/41/165/83/167/21/85/431/411/211 2/21234 LOADLOADF ixed-FreeSimple-SimpleFixed-SimpleFixed- FixedCompression (Column) LoadTension (Pulling) LoadInchesmmInchesmmInchesmmInchesmmABCD 1025420508287114010161538130762421067601 5242050840101657144880203240101680203211 3287016040644511439022861273226180457250 1270100254014135812005080551397110279415 6396222055886015241203048170431824060966 5165113033021844674260660470177814035561 9850292807112751905150381021253853007620 8020321604064226574032081288521591704318 2406096340863690228618045722556477360914 4952413190482627068583809652512710254143 5620508256355012707118031002540358897017 789925151403556307626015248521591203048 Engineering Guidelines for Lead ScrewsColumn Loading Capacities Chart for Lead Screws and Ball

6 ScrewsUse the chart below to determine the Maximum Compression load for Screw Shaft. Usually, screw operated in tension can handle loads up to the rated capacity of the nut, providing the screw length is within standard lengths. End supports have an effect on the load capacity of screws. The four standard variations are shown below with corresponding rating adjustments. Find the point of intersecting lines of load (horizontal) and length (vertical) to determine the minimum safe diameter of screw. If loads fall into dotted lines, consult : DO NOT EXCEED ball nut capacity. Curves for the screw diameters shown are based on the smallest root (minor) diameter of the standard screws within the nominal size Motion.

7 LASH/BACKLASH The axial free motion between the ball nut and screw; a measure of sys-tem stiffness. BEARING BALL CIRCUIT The closed path of recirculating balls within the ball nut assembly. A multiple circuit nut with two or more individual circuits has a greater load carrying capability than a single circuit ball nut assembly of the same diameter. CYCLE The complete forward and reverse motion of the screw (or nut) when moving the load . One cycle is equiva-lent to two load carrying strokes (one forward and one backward). DIAMETER MAJOR The outside diameter of the ball bearing screw shaft. In dealing with ball bearing screws, this is the basic measurement.

8 DIAMETER MINOR (ROOT) Diameter of the screw measured at the bottom of the ball DIAMETER - PITCHThe nominal diameter of a theoretical cylinder passing through the centers of the balls when they are in contact with the ball bearing screw and ball nut tracks. EFFECTIVE BALL TURNS The number of ball groove revolu-tions within the ball nut body; a ball nut with seven effective ball turns will have a higher load carrying capability than one with five, all other charac-teristics being equal. LEAD The axial distance a screw travels during one revolution. LEAD TOLERANCE The maximum variation from nomi-nal, measured in inches per foot, cumulative.

9 load CARRYING BALLS The balls in contact with the ball grooves of both the nut and the screw for load carrying purposes. RIGHT HAND THREAD The direction of the threads on the screw shaft causing the ball nut to travel away from the end viewed when rotated in a counter clockwise direction. SCREW STARTS The integral number of independent threads on the screw shaft; typically one, two or four. GOTHIC (OR OGIVAL) GROOVE A ball track cross-section shaped like a Gothic arch. CONFORMITY RATIO Ratio of the ball track radius to the ball diameter. CONTACT ANGLE Nominal angle between a plane per-pendicular to the screw and a line drawn between the theoretical points of tangency between a ball and the ball tracks and projected on a plane passing through the screw axis and the center of the ball.

10 The angle at which the ball contacts the Contact General DefinitionsBall NutAxial Lash(Backlash)ScrewShaftMajorScrew (Root) NutGothic Form(Ogival)Ball Screw45 Contact 87 Lead and Ball Screws Thrust load (Concentric)Off Center load (Eccentric)Side load (Radial)020406080901000246810% of Ball Screw AssembliesNot FatiguedMillion Inches of TravelRated life (B )Average Life10 Ball Bearing Screw Assembly life vs. TravelRIGIDType CType AType AType CType AType CType C(least support)FREESIMPLERIGIDSIMPLERIGIDRIGIDS IMPLERIGIDRIGIDRIGIDANNEALED ENDS A manufacturing process which removes brittleness while softening screw stock to allow for machining of end journals.