SKF Roller Screws

Critical rotating speed forscrew shaftsThe shaft is equated to a cylinderthe diameter of which is the root diameter of the thread. The formulasuse a parameter the value of whichis dictated by the mounting of thescrew shaft (whether it is simplysupported or fixed). As a rule the nutis not considered as a support of thescrew shaft. Because of the potential inaccuracies in the mounting ofthe screw assembly, a safety factorof .80 is applied to the calculated critical speeds.Calculations which consider the nutas a support of the shaft, or reducethe safety factor, require practicaltests and possibly an optimization ofthe design (1).Permissible speed limitThe permissible speed limit is thatspeed which a screw cannot reliablyexceed at any time. It is generallythe limiting speed of the recirculationsystem in the nut. It is expressed asthe product of the rpm and thediameter of the screw shaft (in mm).The speed limits quoted in this catalogue (see page 64) are the maximumspeed that may be applied throughvery short periods and in optimizedrunning conditions of alignment, lightexternal load and preload with monitored lubrication. Running a screwcontinuously at the permissible speedlimit may lead to a reduction of thecalculated life of the nut mechanism.!High speed associated with high loadrequires a large input torque andyields a relatively short nominal life (1).In the case of high acceleration anddeceleration, it is recommended toeither work under a nominal externalload or to apply a light preload to thenut to avoid internal sliding duringreversal. The value of the preload ofscrews submitted to high acceleration is that preload which ensuresthe rolling elements do not slide (1).Too high a preload will create unacceptable increases of the internaltemperature.The lubrication of screws rotatingat high speed must be properlyconsidered in quantity and quality.The volume, spread and frequencyof the application of the lubricant (oilor grease) must be properly selectedand monitored. At high speed thelubricant spread on the surface ofthe screw shaft may be thrown off bycentrifugal forces. It is important tomonitor this phenomenon during thefirst run at high speed and possiblyadapt the frequency of re-lubricationor the flow of lubricant, or select alubricant with a higher viscosity.Monitoring the steady temperaturereached by the nut permits thefrequency of re-lubrication or the oilflow rate to be optimized.Oil lubricationA centralised recirculating oil systemis ideal because it continuouslychanges the oil in the nut withcooled filtered oil from the reservoir.This system is prescribed when thetemperature is likely to affect thepositioning accuracy. The flow of oilcan be regulated to optimise filmthickness and removal of heat.Selection of oilMineral oil normally used to lubricateother rotating parts such as bearingsand gears may be used for the screw.The viscosity of the oil is defined bythe speed, running temperature andload. The oil should have a viscosityof 100 ISO at the running temperature. An increase in viscosity or speedwill increase the running temperature. At low speed ( 10 rpm) theviscosity should be 200 ISO at therunning temperature. Under heavyload an EP additive to improve thefilm strength is recommended.Corrosion resistant and stabilizingadditives may also be used toadvantage.Grease lubricationWhere oil lubrication is not practicable, the grease recommended forthe support bearings of the screwmay also be used for the screw.After a few full strokes the greasewill be spread evenly over the usefulthreaded length of the screw shaft,which will also help to protect thescrew against corrosion. Howeverthe grease is open to the air.To prevent dirt which may fall on itfrom entering the nut, wipers shouldbe mounted in each end of the nut.Also the grease on the screw shaftwill age more quickly than that inthe screw support bearings so morefrequent regreasing is needed,especially in a dirty environment.If the screw cannot be dismountedand cleaned before regreasing it isnecessary to thoroughly clean theold grease from the screw shaft.This can be done with a spatula andthen with a clean fluff proof cloth.To clean even more a cloth wettedwith a solvent such as white spiritmay be used. We do not adviseusing brushes to remove old orspread new grease (risk of bristlescoming out). Apply also the newgrease through the nut to push outthe old from inside the nut.The grease type is defined above allby the operating temperature, environment and load on the screw.Speed, starting torque and chemicalcompatibility may also be taken intoconsideration.Normally bearing greases of NLGIconsistency 2 are used. A greasewhich is too hard at low temperaturemay restrict rotation or one which istoo soft at high temperature may runoff.Selection of greaseLithium base greases are generallysuitable for use from -30 C to 110 C.A few can be used to 150 C.Lithium base greases are virtuallyinsoluble in water and very workstable. However they absorb largequantities of water when worked toextremes. Other soaps are advisedin the presence of much water.At low speed or heavy load lithiumbase greases with EP additive arerecommended as they adhere wellto the rolling surfaces and are insoluble in water. As a rule EP greasesare recommended for medium andlarge sized screws.(1) SKF can help you to define this value in relation with the actual conditions of service.11

Lubrication intervalThe lubrication interval depends onthe working cycle of the screw andwhether the lubricant is pollutedduring use. General advice is difficult but the following will help you todefine the interval. On start upcheck the grease quality regularly,for instance every month.If the viscosity of the grease samplehas increased, it needs replacing.If the grease sample is darker thannew it may indicate oxidation or thepresence of metallic particles.If it is discoloured, it is probablymixed with water.It is helpful to take samples not onlyfrom the part of the screw whererunning has occured but also fromunused parts of the screw where thegrease acts as corrosion preventer.Quantity of lubricant OilWe advise a quantity of 5 25 cm3/hin shots of 0,1 cm3 as an order ofmagnitude depending on the size ofthe screw and its running conditions. GreaseThe total volume of grease neededfor a new roller screw is the sum ofthe quantities needed for the screwshaft and the nut.The volume of grease necessary forthe screw shaft Zs can be estimatedfrom this formulaZs 4,4 x 10-4 dol1This quantity of grease should bespread over the whole threadedlength of the shaft. The volume ofgrease for the nut Zn is one third ofthe free volume in the nut.The quantity Zn is given in thedimension tables : it should be injected through the lubrication hole whileturning the shaft.Before applying load the nut shouldbe run twice along the completestroke to ensure grease is evenlyspread.Relubrication, see also ‘Greaselubrication’In the case of the existing greasebeing polluted, remove as much of itas possible and apply the samequantity as when first lubricated.If the existing grease is clean add avolume Zn into the nut.Efficiency and back-drivingThe performance of a screw is mainly dependant on the geometry of thecontact surfaces and their finish aswell as the helix angle of the thread.It is, also, dependant on the workingconditions of the screw (load, speed,lubrication, preload, alignment, etc ).The «direct efficiency» is used todefine the input torque required totransform the rotation of one member into the translation of the other.Conversely, the «indirect efficiency»is used to define the axial load required to transform the translation ofone member into the rotation of theother one. It is used, also, to definethe braking torque required toprevent that rotation.It is safe to consider that thesescrews are reversible or backdriveable under almost all circumstances. It is therefore necessaryto design a brake mechanism ifbackdriving is to be avoided (gearreducers or brake).Preload torque : Internally preloaded screws exhibit a torque due tothis preload. This persists evenwhen they are not externally loaded.Preload torque is measured at50 rpm when assembly is lubricatedwith ISO grade 64 oil.Starting torque :This is defined as the torque neededto overcome the following to startrotation :a) the total inertia of all moving partsaccelerated by the energy source(including rotation and linear movement).b) the internal friction of thescrew/nut assembly, bearing andassociated guiding devices.In general, torque to overcome inertia (a) is greater than friction torque(b).12The coefficient of friction of the highefficiency screw when starting µs isestimated at up to double the dynamic coefficient µ, under normalconditions of use.Axial play and preloadPreloaded nuts are subject to muchless elastic deformation than nonpreloaded nuts. Therefore theyshould be used whenever the accuracy of positioning under load isimportant.Backlash elimination can be achievedby fitting oversize rollers in one-piecenut (BRC-BVC).Preload for backlash elimination(TRU/TRK) or optimum rigidity(PRU/PRK - PVU/PVK) can be attained with two half-nuts, pressedtogether or pushed apart.The preload is defined by the valueof the preload torque. The torquedepends on the type of nut and onthe mode of preload (elastic or rigid).Static axial stiffness of acomplete assemblyIt is the ratio of the external axialload applied on the system and theaxial displacement of the face of thenut in relation with the fixed (anchored) end of the screw shaft. Theinverse of the rigidity of the totalsystem is equal to the sum of all theinverses of the rigidity of each of thecomponents (screw shaft, nut asmounted on the shaft, supportingbearing, supporting housings, etc ).111––– ––– –––RtRsRnBecause of this, the rigidity of the totalsystem is always less than the smallest individual rigidity.

Nut rigidity : RnWhen a preload is applied to a splitnut, firstly, the internal play is eliminated, then, the Hertzian elasticdeformation increases as the preload applied so that the overall rigidity increases.The theoretical deformation does nottake into account machining inaccuracies, actual sharing of the loadbetween the different contact surfaces, the elasticity of the nut and ofthe screw shaft. The practical stiffness values given in the catalogueare lower than the theoretical valuesfor this reason. They are determinedby SKF based on the value of theselected basic preload and an external load equal to twice this preload.165 d22 lRs –––––––––l2 (l - l2)for standard steel2d22Rs 165 ––– (N/µm)1for standard steel1Our products have not been developed for use in an explosive atmosphere, consequently we cannot takeany responsibility for their use in thisfield.NOTE : 42 CrMo4, an AFNOR reference is similar to AISI 4140; 100Cr6is similar to AISI 52100.Shaft DesignScrew shaft bucklingShaft rigidity : RsThe elastic deformation of screwshaft is proportional to its length andinversely proportional to the squareof the root diameter.According to the relative importanceof the screw deformation (see rigidity of the total system), too large anincrease in the preload of the nutand supporting bearings yields alimited increase of rigidity and notably increases the preload torque andtherefore the running temperature.Consequently, the preload stated inthe catalogue for each dimension isoptimum and should not be increased.Working environmentIt is possible to deliver roller screwswith one end larger than the outsidediameter, d1 of the shaft. To grind thethread efficiently an undercut at rootdiameter d2 and length l2 is needed.The column loading of the screwshaft must be checked when it issubmitted to compression loading(whether dynamically or statically).The maximum permissible compressive load is calculated using theEuler formulas. It is then multipliedby a safety factor of 3 to 5, depending on the application.The type of end mounting of theshaft is critical to select the propercoefficients to be used in the Eulerformulas.When the screw shaft comprises asingle diameter, the root diameter isused for the calculation. When thescrew comprises different sectionswith various diameters, calculationsbecome more complex (1).Materials and heattreatmentsStandard screw shafts are machinedfrom 42 CrMo4 steel which is surface hardened by induction.Standard nuts are machined in100Cr6 steel which is throughhardened.Hardness of the contact surfaces is56-60 HRc.Most assemblies made of stainlessmaterial have a surface hardness inthe range 42 to 58 HRc : the loadrating of the catalogue thereforemust be derated (1).When d3 1,3 d1SR/BR/TR/PRPh 8 mm,Ph 8 mm,l2 10 mml2 1,4 PhSV/BV/PVPh 1 40 x 2, 50 x 2, 63 x 2All othersl2 9 mml2 14 mmWhen d3 1,3 d1contact SKF(1) SKF can help you to define this value in relation with the actual conditions of service.13

Strength of machined endsThe end machining of a rollerscrew is designed by the customer. Due to the high static anddynamic load ratings of rollerscrews it is important to check thestrength of the machined ends.This checking must be yourresponsibility unless you specifically ask SKF.This simple approach takes intoaccount the different diameters ofthe end where stress concentra-tion factors must be used : it takesinto consideration only axial andtorsional stresses.Zone A. Torsion onlyZone B. Axial torsionalstressesManufacturing precisionThe nominal axial stress caused bythe axial load F is given byThe nominal shear stress τ causedby the drive torque T is given by :16 Tτ ––––π d534Fσ –––πd42This is increased by a stressconcentration factor f5 to give thereal principal stress σpσp f5 σThis is increased by a stressconcentration factor f4 to give thereal shear stress, τpτp f4τAccording to Von Mises the totalstress, σtσt 1,73 τpFor safety σt should be less than67 % of the 0,2 % proof stress of thesteel.If the end diameter d5 includes akeyway of depth e calculate with(d5 - e).The angle of twist of the screw shaftis given by0,0648 Tlθ ––––––do 4The linear positioning error, δ,caused by this twistPh θδ ––––36014as aboveτp f4 τAccording to Von Mises the totalstress σtσt (σp2 3 τp2)1/2For safety σt should be less than67 % of the 0,2 % proof stress of thesteel.Generally speaking, the precisionindication given in the designationdefines the lead precision :see page 15.Lead precision according to ISO(ex. G1 - G5 ).Parameters other than lead precisioncorrespond to our internal standards(generally based on ISO 3408-3class 5).If you require special tolerances(for example class 1 or 3) pleasespecify when requesting a quotationor ordering.

Lead precision according to ISOLead precision is measured at 20 C on the useful stroke lu , which is the threaded length decreased, at each end, bythe length le equal to the screw shaft diameter.G1G3G561223V300p, 12121314161719222529µm Threaded lengthep vuplelulel0mm0 - 315(315) - 400(400) - 500(500) - 630(630) - 800(800) - 1000(1000) - 1250(1250) - 1600(1600) - 2000(2000) - 2500(2500) - 3150(3150) - 134394451596982lu useful travelle excess travel (no lead precision required)lo nominal travells specified travelc travel compensation (difference between ls andlo to be defined by the customer, for instanceto compensate an expansion)ep Mean travel :the line which fitsthe curve best bymethod of leastsquares.vup -v300almv300p300 mmV travel variation (or permissible band width)V300p maximum permitted travel variation over 300 mmVup tolerance over the specified travelmaximum permitted travel variation over theuseful travel luV300a measured travel variation over 300 mmVua Case with value of c specified by thecustomer.measured travel variation over the useful travelCase with c 0 standard version whenno value given by the customer.µmµm Threaded lengthleleluThreaded lengthmmlel0lulecvupepepep-vuavuplmlsl0mmep-15

Recommended assembly procedureRoller screws are precision components and should be handled with care to avoid shocks. When stored out of the shipping crate they must lie on wooden or plastic vee blocks and should not be allowed to sag.Screw assemblies are shipped, wrapped in a heavy gauge plastic tube which protects them from foreign materialand possible pollution. They should stay wrapped until they are used.Radial and moment loadsAny radial or moment load on thenut will overload some of the contactsurfaces, thus significantly reducingits life.YES!NO!Depending on the selected lubricant,it may be necessary to remove thisfilm before applying the lubricant(there may be a risk of non-compatibility).If this operation is performed in apotentially polluted atmosphere it ishighly recommended to proceedwith a thorough cleaning of theassembly.Designing the screw shaft endsAxial loadsRadial loadsAlignmentSKF linear guidance componentsshould be used to ensure correct alignment and avoid non-axial loading.The parallelism of the screw shaftwith the guiding devices must bechecked. If external linear guidanceprove impractical, we suggest mounting the

SKF Group The SKF Group is an international industrial corporation owned by SKF Sweden AB. Founded in 1907, the . In a single start ball screw, the ball diameter is limited to approximate-ly 70 % of the lead : there is only a single