Modeling and Simulation for Material Selection and Mechanical Design Part 16 ppsx

This means, consolidating similar screws due to screwlength, screw diameter, screw head, screw material, and screw surface.. These are screw head, length of thread engagement, screwbody,

Table 14 Estimated Preload Level for Different Unified Screw Types

Preload level (kN), Unified Screw Thread

3=4–10 215.0 29.4 39.2 52.3 49.0 65.4 78.4 104.6 117.6 147.1 161.8 176.5 205.9

7=8-9 298.0 40.8 54.4 72.5 67.9 90.6 108.7 144.9 163.1 203.8 224.2 244.6 285.41–8 391.0 53.5 71.3 95.1 89.1 118.9 142.6 190.2 214.0 267.4 294.2 320.9 374.4

1 1=4–7 625.2 85.5 114.0 152.0 142.5 190.0 228.1 304.1 342.1 427.6 470.4 513.1 598.7

1 1=2–6 906.4 124.0 165.3 220.4 206.7 275.6 330.7 440.9 496.0 620.0 682.0 744.0 868.0

1 3=4–5 1,226 167.7 223.6 298.1 279.5 372.6 447.2 596.2 670.8 838.4 922.3 1,006 1,1742–4 1=2 1,613 220.6 294.2 392.3 367.7 490.3 588.4 784.5 882.6 1,103 1,214 1,324 1,5453–4 3,852 526.9 702.5 936.7 878.2 1,171 1,405 1,873 2,108 2,634 2,898 3,161 3,6884–4 7,148 978 1,304 1,738 1,630 2,173 2,608 3,477 3,912 4,889 5,378 5,867 6,8455–4 11,484 1,571 2,095 2,793 2,618 3,491 4,189 5,586 6,284 7,855 8,640 9,426 10,997

1=4–28 23.5 3.21 4.29 5.72 5.36 7.14 8.57 11.4 12.86 16.07 17.68 19.3 22.5

5=16–24 37.4 5.12 6.82 9.10 8.53 11.4 13.6 18.2 20.5 25.6 28.1 30.7 35.8

7=8-14 328.0 44.9 59.8 79.8 74.8 99.7 119.7 159.5 179.5 224.4 246.8 269.2 314.11–12 428.0 58.6 78.1 104.1 97.6 130.1 156.1 208.2 234.2 292.8 322.0 351.3 409.9

Notes (1) For torque controlled tightening in practice, the preload can be reduced (app  0.7); (2) for utilization of  eq ¼ 90% of R p0.2 , multiply relevant preload by 0.9; (3) yield strength ratio k R ¼ R p0.2 =R m ; (4) for angular controlled tightening, multiply relevant preload by [1 þ 0.3(1  k R ) =k R ].

The difference between initial and residual preload is caused by tact plastification (seating) or relaxation (material creeping, especially athigh temperatures).

con-1 Minimum Initial Preload

The minimum preload required is responsible for the selection of screw size.For a given screw strength and assembly method, the minimum level isgenerated for maximum friction coefficient Therefore, in Table 13 formetric screw thread geometry, a friction coefficient mtot¼ 0.16 is assumed(relevant values of mtotsee Table 4) The listed preload levels are reachedfor yield point controlled tightening Using the legend, preloads for othertightening methods can be calculated To achieve a preload level for a fric-tion coefficient mtot¼ 0.08, multiply relevant value of table by 1.15.From the preload level ofTable 13,with formulae ofFig 16,the corre-sponding torque values can be obtained But for torque controlled tighteningone must remember that the smallest torque corresponds to the smallest fric-tion coefficient and this has to be specified for assembly specification If ascrew with high friction is tightened with the specified torque of low friction,the generated preload is reduced (see aspect 1 of legend from Table 13).Table 13 refers to screws for existing nut thread If thread rollingscrews (Fig 6) are used, the preload level is reduced by some percentagebecause of the higher thread friction (app 5%) For generating nearlythe same preload with thread rolling screws as with same screws for existingnut thread, the tightening torque has to be increased significantly (see alsoRef [63] orFig 67)

Table 14 gives the same information as Table 13 for unified screwthread geometry For designations of screw threads, seeFig 5

How does one find the required minimum initial preload Fp0? As a rule,the initial preload should be at least five times the maximum operating load

of the screw (nfFax) added by 10% for relaxation loss This rule applies tostable threaded fastening systems without creeping effects The initial pre-load must prevent any component from separating (guaranteeing sealingfunction, see also Fig 49; avoiding of increasing load factor f, see also

Fig 25), microsliding (fretting, self-loosening) or significant relaxation(continued preload loss with possible failure in consequence)

2 Boundary Conditions in Practice

For selection of screw size, handling during operation and field maintenance

is an important consideration As an example, a screw of dimension M6 orhigher can normally be hand tightened by workers without danger of

overtightening A screw up to M12 can be tightened=retightened with normalwrenches and moderate manpower Screw dimensions between M6 and M12can be used by nearly every person without special qualification=trainingand=or special equipment.

The design engineer can always decide if a few large screws or moresmall screws are used to achieve the summarized preload An increasednumber of small screws has the advantage of better stress homogeneity inthe components, better sealing of flanges, reduced local separating of com-ponents with low stiffness under operating load But a multi-screw-fasten-ing-system needs a detailed calculation of the loading of each particularscrew and a defined tightening sequence during assembly

Screws, which need exact preload, should be tightened by yield pointcontrol or angular control (see alsoFig 51)

Finally, the requirements from deproliferation have to be met Thenumber of different parts which have to be purchased, stored, and managed,has to be minimized This means, consolidating similar screws due to screwlength, screw diameter, screw head, screw material, and screw surface

C Determination of Screw Geometry

If the screw thread size is known, several additional geometry details have to

be determined These are screw head, length of thread engagement, screwbody, thread length, and other design options This chapter shows the fun-damental aspects for design decisions

1 Thread Engagement

If the design principle fromFig 2is valid, the thread engagement requires aminimum value temin, and thread stripping of screw or nut cannot happen

Figure 33 points out the result from calculations regarding the VDI

2230 guideline [70] for metric thread series (thread standard, see Fig 4)

The diagram illustrates the relative minimum thread engagement temin=dover tensile strength of nut thread component Rmn for different propertyclasses of screw Details are printed in the diagram Generally speaking,the required length of thread engagement increases with increasing screwstrength Rmsand decreasing nut strength Rmn This diagram has two dimen-sions of interpretation: for thread engagements higher than the relevant

temin, no thread stripping will occur and in any case the screw shank will fail(direction of ordinate-axis) If the relevant point for temin on the selectedhyperbolic curve is located in the tangential section, the screw thread willstrip for engagements smaller than temin If the relevant point for temin is

threaded cross-section with area As and the unthreaded cross-section witharea Abresp A2(area of cross-section with flank diameter d2).

The length of a threaded shank with rolled thread flanks as shown in(a) is limited by the length of the rolling die for screw production A fullshank (b) has a constant outer diameter in the range of the nominal screwdiameter d Such a screw possesses good self-centering behavior throughholes An exactly defined centering function can be realized with anincreased shank (c) A reduced shank (d) often is an optimum betweenscrew-weight, -cost, and -function, because the reduced shank has a dia-meter in the range of thread flank diameter d2, so the screw production linecan be made effective A wasted shank (e) gives a high screw resilience withlow additional screw force under loading; here the body diameter dBshould

be made as long as possible (a guiding diameter is necessary under head andthe transition between different diameters has to be designed with large radiifor avoiding of stress concentrations) As a guideline, a shank type (a) or (d)should be taken whenever possible

The clamping length is the distance between head support and start ofthread engagement and the plastification length is the length of free shankunder preload with smallest cross-section Asor Ab Therefore, lcis the same

Figure 34 Clamping length lc and plastification length lp of threaded fasteningsystem from Ref 18.)

for all screw types (a)–(e) In contrast, the plastification length varies from

lp¼ lcat type (a) to smaller values at types (b)–(e) By reason of the cant area difference between Asand Abresp A2at the same screw, only thesmallest cross-section will plastify under tensile load; this smallest cross-sec-tion comes to failure before the other cross-section gets plastified dependent

signifi-on the materials ratio of Rp0.2sover Rms

Figure 35 Basics of screw drive selection

preload because they cannot provide high torque values which can be mitted reliably between bit and screw.

trans-The most common screw drive globally is the hexagon geometry.This is important for components which have to work and must berepaired in areas without technical experience This drive type is suitablefor high torque values if there is only a small clearance between bit=wrenchwrench and screw and if the drive has no damage Using an open wrench

as a rough estimation, only half of the torque compared with a ring ner can be applied with reliability The reason is that when using an openwrench, only two flanks are used for torque transmission Since six driveflanks and a small contact angle between bit and screw for the line contact,the hexagon drive may lead to damaging the surface of the screw, espe-cially if the screw is coated for corrosion protection or if worn bits areused InFig 35, these aspects lead to a sum of 9 assessment-points fromthe 20 possible

span-A significant improvement of drive torque loading capacity and bility is achieved with 12 flanks (bihexagon and triple-square drive geome-tries) A bihexagon drive geometry is created by two hexagon drives,which have the same center point and an angular misfit of 308 A triplesquare drive geometry is created by three square contours, which have thesame center point and an angular misfit of 308 each

relia-A hexalobular drive geometry [established by Textron-Camcar underthe designation TORX#] consists of one (small) convex and one (large) con-cave contour radius, which are alternately combined [22] This leads tosmooth contact pressure between screw and bit as well as small-sized outerbit diameters for compact design structures There is no significant differ-ence in using this design compared to bihexagon or triple square, except thatthe same maximum drive diameter, the hexalobular drive geometry has alower drive section modulus against torsional failure Triple square orbihexagon drives should be used

For the internal drive configurations, the same comments are valid.Compared to the external configurations with the same head diameter,the drive flanks are smaller and the internal configurations are stressed to

a higher level for same torque transmission The bit is much smaller which

is very positive for the accessibility In most cases for internal bihexagon, ple square or hexalobular drive, the bit determines the torque limit, not thedrive of the screw Internal drive configurations usually have lower weight

tri-of the screw head than external drives, but internal drives can lead to headstripping under preload, if their bit-engagement is too deep On the otherhand, a minimum bit engagement is necessary for reliable assembly process.These two influences determine the height of head for screws with internaldrive

Slotted screws are only relevant for applications with low requirementsfor screw tightening They have a cam-out-reaction under torque loadingand the blade of the screw driver can have a radial misalignment, whichleads to damage of screw, screw driver and possibly of component surface.Cross-recess drives are an obvious improvement over the slotted screws inlow torque applications like screws for fastening wooden constructions orplastic components They provide a radial alignment between screw anddriving bit, but the negative cam-out-reaction is significant The life time

of cross-recess bits is quite short

Of course, there exists many other drive systems for special ments, such as Square drive, Multispline, Hexapol#, Triwings#, Clutch-

require-Figure 36 Contact conditions of high torque screw drives (From Ref 17.)

type#, Torx-Plus#, or Polydrive#, which often are trademarks of differentcompanies.

Figure 36 demonstrates the contact conditions of high torque screwdrives fromFigure 35in a more detailed manner In any case, the tolerancesituation is important for the torque loading limit of the drive The clearance

in Fig 36 is oversized in order to emphasize that all screw drives have singlecontact lines at each drive flank, if they are undeformed (only contact points

in drawn cross-sections)

The applied torque Ttotleads at each drive flank to a circumferentialforce Fc, which can be divided into a normal part Fn(torque transmission)and a tangential part Ft(contact sliding and in consequence flank wear) For

an ideal drive geometry, this Fccan be calculated as shown in Fig 36.Between Fcand Ft, one can measure the contact angle E This value is

308 for hexagon and bihexagon drive, 458 for triple square (see alsoFig 63)

and about 608 for hexalobular drive geometry A small contact angle meanshigh contact sliding under torque loading This is the reason for surfacedamaging of the screw area engaged to the bit as well as the reason for wear

of the bit flanks

Figure 36confirms that a bihexagon drive has the same contact ditions as a hexagon geometry, but the increased number of engaged flankslowers the Fcat each single flank The triple square and hexalobular drivesystems have an increased contact angle, so they should be taken as adesigned screw drive system today, if no advantages of other drive systemsare predominant

con-If the clearance between bit and screw drive contour is too large, thebit life time decreases significantly and the danger of screw drive damagingoccurs

Often for small-volume-designs, the space for screw head and theaccessibility for bit are limited.Figure 37compares the space requirements

of three screw head designs with hexalobular drive type for same thread meter d and same support diameter da Part (a) refers to an external config-uration, which is characterized not only by high stiffness of the screw head,but also by large height requirements The bit for driving the screw normallyhas a largest diameter up to 2.0d as the head support diameter da

dia-If using a standard design with internal configuration (b) the height ofscrew head is reduced to80% of (a) Also, the size of the screw drive flanks

is reduced to only 60% of (a) This can cause problems if the screw hashigh material strength and if the screw is tightened to high preload levelbeyond the screw material yield limit In this case, the cross-section of thedriving bit exceeds its fatigue limit, so that the life time of the bits isdecreased drastically Another aspect of internal drive configuration is theratio of screw head height and length of bit engagement This ratio has to

of head is only 0.7d, no head stripping occurs under preload due to the son of the conical head-shank-transition The large length of bit engagementguarantees a high assembly process capability The large size of screw driveflanks leads to a long bit life time for any tightening method The internalconfiguration offers an easier drive accessibility by a small bit diameter com-pared to the external configuration of (a).

rea-Another important design aspect of screw head is the type of supportarea Figure 38 displays three established types of support area betweenscrew head and clamped part Each type has its own calculation for theeffective bearing diameter Deb[72] This diameter Debrepresents the virtualdiameter, where the circumferential force produced by the contact frictioncan be concentrated for calculation; it influences the head frictional torque

Thdirectly (seeFig 16)

A plain support type is used as a standard; it is easy to manufacture andrequires no special geometrical matching of screw and clamped part Largehead support diameters da are suitable for low surface contact pressure(see also Figure 39) and for covering large clearance holes Countersunk-and ball-section-support types provide a centering function between screw

Figure 39 Required relative support diameter for given maximum contactpressure (From Ref 16.)