b.3.2 fastening with metal screws, farbig
DESCRIPTION
fastingTRANSCRIPT
Fastening with metal screws
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B.3
.2
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Contents
1. Introduction
2. Requirements for screwedjoints
3. Basic types ofscrewedjoint3.1 Joint with self-tapping screws
3.1.1 Thread-forming screws
3.1.2 Thread-cutting screws
3.2 Joint with quick-fix nuts
3.3 Joint with metric screws
3.3.1 Joint with screw and nut
3.3.2 Joint with threaded boltsanchored in plastic
3.3.3 Joint with threaded insertsanchored in plastic
4. Critical parameters for screwed joints4.1 Screwed joint
with thread-forming screws
4.1.1 Nominal screw diameter dand screw engagement length L
4.1.2 Thread bite t
4.1.2.1 Thread depth h4.1.2.2 Thread pitch P4.1.2.3 Thread angle a
4.1.2 .4 Receiving hole diameter dK4.1.2.5 Outside diameter D of moulded bosses4.1.3 Shear strength Ks and tensile strength Kz
of the plastic4.1.4 Relaxation modulus Er of the plastic4.2 Screwed joint
with thread-cutting screws
4.3 Screwed joint with threaded inserts
and threaded bolts
5.
4
5
5
5
5
8
8
8
8
9
10
11
11
11
6.
Behaviour ofscrewedjointsunder steady stress
5.1 Joint with self-tapping screws
5.2 Joint with threaded inserts
Permissible stresses for screwed joints6.1 Driving torque MA6.2 Axial force Fperm.
12
12
13
13
13
13
7. Securing screwedjoints7.1 Joint with thread-forming screws
7.2 Joint with thread-cuttingand metric screws
8. Design notes
8.1 Joint with self-tapping screws
8.2 Joint with metric threaded inserts
and metric threaded bolts
9. Calculation examples
10. Applications
11. Explanation ofsymbols
12. Literature
14
14
15
15
15
16
17
19
21
21
1. Introduction
For detachable fastening of plastic components, metalscrewed joints are a frequently employed option. These
provide a high-strength joint capable of withstandingcontinuous stresses safely even at relatively high service
temperatures. By using additional sealing elements
(O-rings for example), leak-tight joints can also beobtained.
2. Requirementsfor screwedjoints
Screwed joints are designed to fix components perma
nently in a certain position relative to each other. Toachieve this, a pre-stressing force is required which is
applied by tightening the screws and must be maintainedat an adequate level for a long period of time. This pre-stressing force must be greater than the forces occurringin the normal functioning of the components and must
also be greater than random stresses which could arise
for instance in transporting or handling the parts. For
this reason, metal screws are normally oversized so that
strength testing is not generally required. Strength testingof the screwed joint is however necessary if the metalscrew is anchored directly in the plastic part and hencethe lower mechanical properties of the plastic determinethe strength of the joint.
Screwed joints should be easy and cheap to make. This
requirement is particularly well fulfilled by screws which
tap their own thread in the plastic part (self-tappingscrews).
HostaformAcetal copolymer (POM)
HostacomReinforced polypropylene (PP)
= registered trademark
3. Basic types ofscrewed joint Fig. 2: Thread designs of different thread-formingscrews
3.1 Joint with self-tapping screws
Injection moulding of the internal thread increases mould
costs and generally lengthens cycle times. For this reason,
thread-forming and thread-cutting screws are an advan
tage. They are screwed into a cylindrical receiving hole
so forming the internal thread.
3.1.1 Thread-forming screws
For forming the internal thread by mechanical displacement of material (fig. 1), screws with a sheet metal screw
thread as specified in DIN 7970 or wood screw thread as
specified in DIN 7998 are suitable.
Fig. 1 : Screwed joint with thread-forming screw
In addition, there are a whole series of special screw
designs developed for joining plastic parts, for example
ABC Spax screw
Fa. Altenloh, Brinck & Co., D-58256 Ennepetal
PT screw
Fa. E. Jäger GmbH & Co., D-57319 Bad Berleburg
Plastite screw
Vertrieb]. H. Krumb, D-61440 Oberursel
-v-v^r^r^rvr_jj P i*
sheet metal screw DIN 7970
AA7V_J\_A_^V
wood screw DIN 7998
/T^A_J_J\_^L
special screw for plastic parts(e. g. Spax screw)
d ! outside diameter ( without taking into account themanufacturing tolerances for the nominal diameter d)
d2 receiving hole diameterP pitch
thread angle
These special designs differ from sheet metal and woodscrews in having a smaller receiving hole diameter and
consequently a greater thread depth. The thread angleof these screws is 30 to 45 compared with the 60 forsheet metal screws. Fig. 2 shows the thread designs ofvarious thread-forming screws.
An essential requirement for this method of forming theinternal thread is that the plastic is sufficiently tough,i.e. that it will accept plastic deformation without crack
ing. Furthermore, the stressed (deformed) regions of the
plastic parts should not be liable to environmental stress
cracking. Hoechst engineering plastics satisfy this requirement.
3.1.2 Thread-cutting screws
For less ductile plastics such as the reinforced partiallycrystalline thermoplastics, thread-forming screws are not
so suitable. Thread-cutting screws on the other hand can
be used to advantage. The most suitable are thread-cuttingscrews as specified in DIN 7513 and sheet metal screws
with cutting notches or a cutting edge in the first turns ofthe thread (e. g. Knipping notched screw, Fa. A. KnippingGmbH, D-51643 Gummersbach), fig. 3.
Fig. 4: Screwed joint with quick-fix nut (principle)
Fig. 3: Sheet metal screws with cutting notches inthe first turns of the thread (left) and with a cuttingedge (right)
3.2 Joint with quick-fix nuts
In addition to direct screwing into the plastic part, sheetmetal screws may also be used in combination with
quick-fix nuts (e. g. A. Raymond, Befestigungselemente,D-79539 Lörrach; United Carr GmbH, D-67677 Enken-
bach-Alsenborn; Mecano Simmonds GmbH,D-69123 Heidelberg). These parts made from springsteel have two claws matched to the thread pitch, fig. 4.
^%rv~^
^^ ^<^
As the screw is driven in, the claws press against thethread root and so permit a vibration-resistant, self-
locking joint. If the nuts are suitably designed, they can
be pre-fitted to the plastic part as captive fasteners.
3.3 Joint with metric screws
3.3.1 Joint with screw and nut
Fig. 5: Screwed joint with screw and nut
For direct screwing into the plastic part, metric screws
as specified in DIN 13 are not so suitable because of their
relatively shallow thread depth. They should therefore
preferably be used in combination with metal nuts or
threaded inserts.
Fig. 6: Screwed joint with additional supporting sleeve Fig. 7: Screwed joint with moulded-in or subsequentlyto take the pre-stressing force installed threaded bolt
Fig. 5 shows a screwed joint with screw and nut. To facili
tate assembly, the nut is frequently snap-fitted into an
undercut recess as a captive fastener.
To prevent plastic parts deforming as a result of the screw
pre-stressing force with consequent loss of pré-stress, a
thin metal sleeve can additionally be fitted in the receivinghole, fig. 6. Its length should correspond to the sum of the
wall thicknesses of both plastic parts. This solution is an
advantage for parts exposed to temperature variations.
3.3.2 Joint with threaded bolts anchored in plastic
Fig. 7 shows a screwed joint with threaded bolt. Thethreaded bolt is either inserted in the mould before injection moulding and moulded in or embedded by ultra
sonic means into a receiving hole after moulding. In
the latter method, the outer contours of the bolt in the
seating region are profiled to ensure good anchorage in
the plastic part.
3.3.3 Joint with threaded inserts anchored in plastic
In the screwed joint shown in fig. 8, too, the threadedinsert is either moulded in or subsequently installed in
the plastic part, e. g. by ultrasonic means.
Fig. 8: Screwed joint with moulded-in or subsequentlyinstalled threaded insert
Fig. 9: Examples of suitable threaded inserts for:
- moulding-in a
- ultrasonic or heated tool insertion (a), b, c
- mechnical anchorage d, e (expansion-type),f (with external thread)
b, d, f sold by Kerb-Konus-Vertriebs-GmbH,D-86854 Ambergc, e sold by Böllhof & Co., D-33649 Bielefeld
threaded insertas specified inDIN 16 903
4. Critical parametersfor screwedjoints
A vital factor in determining the strength of a screwed
joint is the pre-stressing force applied by driving in thescrew. In this section, the parameters which determine the
permissible driving torque (pre-stressing force) and the
permissible forces which may be exerted on the screwed
joint in service are discussed.
4.1 Screwedjoint with thread-forming screws
4.1.1 Nominal screw diameter d and screw
engagement length L
The strength of a screwed joint (pull-out force F or stripping torque M) is directly proportional to a shear-stressed
cylindrical area calculated from the nominal screw diameter d and screw engagement length L, fig. 10.
F,M~jr-d-L
Fig. 10: Screw engagement length L
CM
(1)
Fig. 9 shows examples of different threaded inserts.cylindrical area
-T-d-L
The screw engagement length L is the length of screw
engaged in the plastic from the lowest fully engagedthread to the top of the receiving hole.
4.1.2 Thread bite t
The strength of a screwed joint is directly dependent on
the thread bite t, fig 11. A deep thread bite means highstrength. With thread-forming screws, the thread biteobtained when the screw is driven in depends on the
following dimensions:
Fig. 11: Thread bite t
A ' A
4.1.2.2 Thread pitch P
The thread pitch P which is the distance between two
consecutive turns of a thread determines - along withthread depth h - the space available to accommodate the
displaced plastic - i.e. a high thread pitch P permits a
correspondingly deep thread bite t.
4.1.2.3 Thread angle a
When the screw is driven into the cylindrical receivinghole, the thread penetrates the plastic like a wedge and so
forms a mating thread. The penetration depth and hencethe thread bite t (fig. 11) increases with decreasing thread
angle a.
Table 1 : Comparison of critical screw dimensions for
screws with a nominal diameter of 3.5 mm
^\^ Dimension
Screw type ^v
sheet metal screw
wood screw
ABC Spax screw
PT screw
Plastite screw
Outsidediameter
d,[mm]
3.5
3.5
3.5
3.5
3.4
Thread
depthh
[mm]
0.45
0.55
0.7
0.78
0.45
Thread
pitchP
[mm]
1.27
1.6
1.63
1.58
1.27
Thread
angleK
[]
60
60
40
35
60
4.1.2.1 Thread depth h
The greater the thread depth
h = d,-d2(2)
fig. 11, the greater the space available to accommodate the
displaced plastic. A high thread depth h permits a deepthread bite t and hence high joint strength.
4.1.2.4 Receiving hole diameter dK
The receiving hole diameter dK has a decisive influence
on the achievable thread bite t which determines the
strength of the joint. Fig. 12 plots the curve for pull-outforce F and stripping torque M as a function of receivinghole diameter. Suitable receiving hole diameters are shownin table 2.
Fig. 12: Pull-out force F and stripping torque M as a
function of receiving hole diameter ds.
F,M
ds
Table 2: Recommended receiving hole diameter dK for
thread-forming screws
4.1.2.5 Outside diameter D ofmoulded bosses
With some screwed joints, it is necessary to drive the
thread-forming screw into a relatively thin flat plasticpart, fig. 13 a. In this case, the greatest possible threadbite should be obtained because this ensures the greatestresistance against expansion of the plastic part as the
screw is driven in. More frequently, however, a boss is
provided to take the screw, fig. 13 b. In this case, thethread bite t additionally depends on the outside dia
meter D; t diminishes with decrease in D.
Kg. 13: Receiving holes for thread-forming screws
Screw type^^^^
^^\^Material
Hostaform C 52021
Hostaform C 27021
Hostaform C 13021
Hostaform C 13031
Hostaform C 13021 RM
Hostaform C 9021
Hostaform C 9021 K
Hostaform C 9021 M
Hostaform C 9021 TFHostaform C 9021 GV 3/10
Hostaform C 9021 GV 3/20Hostaform C 9021 GV 3/30
Hostaform C 2521
Hostaform C 2552
Hostaform T 1020
Hostaform S 27063
Hostaform S 27073
Hostaform S 27064
Hostaform S 9063
Hostaform S 9064
Hostalen PPN 1060
Hostacom Ml U01
Hostacom M4 U01
Hostacom M2 N01
Hostacom M4 N01
Hostacom M2 N02
Hostacom G2 N03
Hostaform C 9021 GV 1/30
Hostaform C 9021 GV 1/40
Hostacom G2 N01
Hostacom G3 N01
Hostacom G2 N02
Sheet metal
screw,
wood screw,
Plastite screw
d
rk<
<
\\J\N
\\\N\\
$>, v
K = 0.8 d
f] .
rTn(
ii /N ! ^
!____.
11
- dfC
dK = 0.85
$
1J>
~*\\\\^N>$S\\ \L\
d
ABC Spaxscrew,
PT screw
dK = 0.75 d
dK = 0.8 d
S
D'
4
r>
**
1
The outside diameter D also has a direct influence on the
strength of the joint. Depending on the outside diameter
D, the following types of failure may be observed whenthe joint is overloaded, fig. 14:
- shearing of the internal thread- fracture of the boss in the circular area under
tensile stress.
The type of failure which occurs is determined by screw
engagement length L. If, as recommended, the screw
engagement length is
L = 2.5d (3)
*d = nominal screw diameter
then in the event of overloading, the boss fractures up to
an outside diameter of D < 2.5 d. If, on the other hand
D > 2.5 d, then the internal thread shears (fig. 14).
Fig. 14: Types of failure in screwed joints withmoulded bosses
a Shearing of the internal thread in the cylindricalarea AI = n d L
b Failure of the boss in circular area
A2=-f-(D*-<P)
A,
4.13 Shear strength Ks and tensile strength Kzof the plastic
The strength of the screwed joint is proportional to thematerial characteristic values KS (shear strength) and KZ(tensile strength) determined directly at the joint.
These values correspond to the shear strength and tensile
strength 0B of the material but vary in magnitude sinceadditional influences are involved such as:
- multi-axial stress condition in the moulded boss- notch effect at the root of the internal thread.
Using the material characteristic values Ks and Kz given in
table 3, the strength of the joint can be roughly estimatedin advance.
Table 3 : Characteristic values for shearing of the internalthread Ks and fracture of the boss Kz (determined for
dK = 0.8 d, L = 2.5 d, D = 2.5 d, and D > 4 d)F = pull-out force, D = outside diameter of the boss,d = inside diameter of the screw, L = screw engagementlength
^x. Material character-^v istic
^SVV values
^S.Material _\Hostaform C 52021
Hostaform C 27021
Hostaform C 13021
Hostaform C 13031
Hostaform C 13021 RM
Hostaform C 9021
Hostaform C 9021 K
Hostaform C 9021 M
Hostaform C 9021 TF
Hostaform C 9021
GV 3/10Hostaform C 9021
GV 3/20
Hostaform C 9021
GV 3/30
Hostaform C 2521
Hostaform C 2552
Hostaform T 1020
Hostaform S 9063
Hostaform S 27063
Hostaform S 27073
Hostaform S 9064
Hostaform S 27064
Hostaform C 9021
GV 1/30
Hostaform C 9021
GV 1/40
Hostalen PPN 1060
Hostacom Ml U01
Hostacom M4 U01
Hostacom M2 N01
Hostacom M4 N01
Hostacom M2 N02
Hostacom G2 N01
Hostacom G2 N03
Hostacom G2 N02Hostacom G3 N01
KF
Ks~;r.d. r- [N/mm2]
D>4d
20C
47
40
30
50
24
26
28
80C
28
23
18
35
11
14
19
D = 2.5d
20C
40
33
25
46
20
22
24
80C
24
20
15
33
9
12
16
KF
*Z-(D2-d2)D<2.5d
[N/mm2]20C
50
42
32
55
26
26
28
80C
30
25
19
-
14
14
19
10
Fig. 15: Stress distribution
Fig. 16: Reduction factor for the decay in pre-stressingforce with time
100
I
3-d
20
10- 10 101 102 103
Stress duration
10" h 105
a Hostaform Cb Hostacom G3 N01c Hostacom M2 N01
Depending on the magnitude of the outside diameter D
and hence on the type of failure, the joint strength is
determined with Ks or K2. If D > 2.5 d then the internal
thread shears in the event of an overload and the jointstrength is calculated from the characteristic K5 and the
cylindrical area A] (fig. 14a). If D < 2.5 d, the charac
teristic value Kz and circular area A2 are used (fig. 14b).
The decay in pre-stressing force with time can be calculated
approximately from the reduction factor curve shown in
fig. 16. In reality, the relationships are better than that
because owing to friction in the engaging surfaces, some
thing approaching a hydrostatic (tri-axial) stress condition
(all-round compression) exists in which stress relaxation
is reduced.
4.2 Screwedjoint with thread-cutting screws
For this type of joint, basically the same factors applyas for joints with thread-forming screws (see chart 1).However, the boss outside diameter D has less effect
on thread bite t. To calculate joint strength, the charac
teristic values KS and KZ from table 3 are used.
Chart 1: Effect of critical parameters on strength ofscrewed joints with self-tapping screws
Increase in
Nominal screw diameter DScrew engagement length LThread bite t
Outside diameter D of boss
Shear strength KsTensile strength KZStress duration
Effect oni
joint strength
increase t
increase t
increase t
increase t
increase t
increase t
decrease 1
4.3 Screwedjoint with threaded inserts andthreaded bolts
For this type of joint, too, joint strength basically dependson the shear-stressed cylindrical surface (see section 4.1.1)between the plastic part and the threaded insert or bolt.This is calculated from the outside diameter and length ofthe metal insert. Because of the variation in insert profiles(see fig. 9), it is not possible to give a material characteristicvalue KS as for the thread-forming screws. To obtain a
rough estimation of pull-out forces, the shear strengthvalues TB shown in table 4 can be used.
4.1.4 Relaxation modulus Er of the plastic
The pre-stressing force applied during assembly exerts
a compressive stress p on the plastic part in a direction
parallel to the longitudinal axis of the screw (fig. 15).This compressive stress diminishes in the course of time
as a result of stress relaxation.
11
Table 4: Shear strength TB at room temperature
Material
Hostaform C 52021
Hostaform C 27021
Hostaform C 13021
Hostaform C 13031Hostaform C 13021 RMHostaform C 9021
Hostaform C 9021 K
Hostaform C 9021 MHostaform C 9021 GV 3/10
Hostaform C 9021 GV 3/20Hostaform C 9021 GV 3/30
Hostaform C 2521
Hostaform C 2552
Hostaform T 1020
Hostaform C 9021 TF
Hostaform S 9063
Hostaform S 27063
Hostaform S 27073
Hostaform S 9064
Hostaform S 27064
Hostaform C 9021 GV 1/30
Hostaform C 9021 GV 1/40
HostalenPPN 1060
Hostacom Ml U01Hostacom M4 U01
Hostacom M2 N01
Hostacom M4 N01Hostacom M2 N02Hostacom G2 N01Hostacom G2 N03
Hostacom G3 N01Hostacom G2 N02
Shear strength TE [N/mm2]
43
33
36
26
80
18
43
5. Behaviour ofscrewedjointsunder steady stress
5.1 Joint with self-tapping screws
Fig. 17 shows the stripping torque and pull-out forceof screwed joints made with various screw sizes. Curvea is for Hostaform and curve b for the Hostacom grades.The upper limit of the curve b range represents the glassfibre reinforced grades G2 N02 and G3 N01, the lowerlimit the talc-reinforced grades. These loading limits also
apply with good approximation to PT screws, ABC Spaxscrews and Plastite screws, which all give somewhat bettervalues as a rule.
Fig. 17: Failure curves for screwed joints with sheetmetal screws
a in Hostaformb in Hostacom
2.2 2.9 3.5 4.2 4.8 6.3 mm
Nominal screw diameter d
2 4 6 8 10 14
ISO No.
12
5.2 Joint with threaded inserts
Fig. 18 shows the stripping torque and pull-out force forthreaded inserts embedded in Hostaform by various
means. It can be seen that threaded inserts which are
moulded in or ultrasonically installed have the greatestholding power, followed by inserts placed by heated tooland press-fitted inserts.
6. Permissible stresses
for screwedjoints
Experience has shown that it is best to select the permissible driving torque MA and the permissible axial force
Fperm. using the overload curves in section 5.
Fig. 18: Failure curves for screwed joints with threadedinserts embedded in injection moulded parts madefrom Hostaform
a Threaded insert as specified in DIN 16903 Sh. 3, moulded in,and Hit-Sert 2 or Sonic Lok threaded insert, installed ultrasonically or by heated tool.
b Banc-Lok self-locking threaded insert, press-fitted with 0.3 nuninterference and expanded by turning the screw.
c Dodge self-locking threaded insert, press-fitted with 0.05 mminterference and expanded by turning the screw.
aa Threaded insert as specified in DIN 16903 Sh. 3, moulded in,and Hit-Sert 2 or Sonic-Lok threaded insert, installed ultrasonically.
ab Hit-Sert 2 threaded insert, installed by heated tool.
6.1 Driving torque MA
The driving torque MA must be great enough for the
connecting parts to be in full and secure contact and fora sufficiently high pre-stressing force to be created. This
requirement is met if the driving torque MA is selected
as follows:
Hostaform (basic grades and modified grades)MA = 0.25 to 0.3 M (4)
Hostacom
MA = 0.35 to 0.4 M (5)
tt
C/Î
M = stripping torque (figs. 17 and 18).
6.2 Axialforce Fperm.
When the screwed joint is under a constant continuous
stress, experience has shown that about 25 to 30% of the
pull-out force shown in figs. 17 and 18 can be permitted,i.e.
fi
=U 3000
2000
1000
Fpern, = 0.25 tO 0.3 (6)
M3 M4 M5
Internal thread in threaded insert
M6
13
7. Securing screwedjoints Fig. 19: Compressive stresses arising fromtemperature variations
7.1 Joint with thread-forming screws
When thread-forming screws are driven into a cylindricalreceiving hole, the material in the thread region under
goes plastic deformation and in adjacent outer regionselastic deformation. This results - as with press-fit joints(see also B.3.4 Design calculations for press-fit joints)- in a radial pressure pr being exerted on the screw
(fig. 19). This decreases with time according to the relaxation modulus Er but does not reach zero. The radial
pressure ensures a friction grip between the screw and
plastic which generally prevents accidental detachmentof the joint even if the axial screw force is zero.
A decrease in the axial force produced when the screw isdriven in is particularly likely when stressed plastic partsare exposed to temperature variations. When there is a
temperature increase, expansion of pan A is largely prevented by the metal screw, fig. 19. The compressivestress p# thereby produced partly relaxes and partlycauses lateral displacement of the material (<=> fig. 19).
On subsequent cooling, part A is free to contract andso the compressive stress in an unfavourable case mayreturn to zero.
To maintain a friction grip in the axial direction when the
joint is exposed to temperature variation, it is necessaryto incorporate spring elements into the joint. Spring lockwashers as specified in DIN 137 (fig. 20) and DIN 6769
(fig. 21) are suitable.
Spring rings as specified in DIN 127 produce a relativelyhigher loading pressure because of their smaller contact
area and should therefore only be used in conjunctionwith a washer.
O-rings and other elastic sealing elements can be used incombination with spring elements.
Fig. 20: Spring lock washer
Type A dished Type B wavy
Standard designation for a spring lock washer, type A size 1 0 :
spring lock washer A 10 DIN 137
Fig. 21 : Conical spring lock washer
-*\S
burr-freer
i-O T3
1
Standard designation for a conical spring lock washer of nominalsize 8, made from spring steel (F St): conical spring lock washerDIN 6769-8-F St
14
7.2 Joint with thread-cutting and metric screws O 7")é?çfpt7 ÎÎOteS
In the case of thread-cutting and metric screws, accidental detachment of the joint is prevented by the frictioncreated through the axial force of the screw. When the
joint is exposed to temperature variation, the axial screw
force is maintained by spring elements (see section 7.1).
Another successful way of securing the screw is to introduce an adhesive into the screw thread.
Practical trials have shown that joints with thread-formingscrews can be detached and reassembled up to about15 times without loss of strength. This assumes that the
screw is always driven into the same internal thread. Thisis normally the case when the screw is driven by hand.
With thread-cutting screws on the other hand, frequentdetaching and reassembling of the joint is not recom
mended because the internal thread may be broken.
Joints with threaded inserts or threaded bolts can bedetached any number of times.
8.1 Joint with self-tapping screws
For self-tapping screws, cylindrical receiving holes withthe dimensions shown in fig. 22 are recommended. Some
times in practice, triangular or square holes are also provided to minimize the screwing torque. This solution can
be an advantage for hard, brittle plastics with unfavour
able sliding properties (e.g. polystyrene, thermosets etc.).Thread bite is slightly reduced but this can be offset bya greater screw engagement length. In deciding on thelocation of bosses, care should be taken to avoid materialaccumulation (fig. 23).
Fig. 22: Receiving hole for self-tapping screws Fig. 23: Avoiding material accumulation in mouldedbosses
a on a wall, b in a corner
0.80 to 0.85 d
sink marks sink marks
Alternative designs to avoid material accumulation
r~
alternatives alternative 2
15
8.2 Joint with metric threaded inserts andmetric threaded bolts
In dimensioning bosses to take threaded inserts andthreaded bolts it should be remembered that, when theinserts are moulded in, a minimum wall thickness is
required to prevent cracking. An adequate wall thicknessis provided if the outside diameter D is at least 1.6 timesthe diameter dB (fig. 24), i.e.
When a threaded insert is to be ultrasonically installed, a
hole interference x of about 0.4 mm should be provided.In each case, care should be taken to ensure that the topedge of the threaded insert is level with or projects abovethe top edge of the boss so that the axial force is introduced directly into the threaded insert, fig. 25.
D ä 1.6 dB (7)
Fig. 24: Boss for a threaded insert
dB = outside diameter of threaded insert
Fig. 25: Arrangement for a threaded insert
=Q
!"t
0
** 1.6 to 1.8 ds-V
*
\
§S
dB
dfl
A
-x
s^
<^
*-
; ^
§svn
16
9. Calculation examplesExample 1
The top of a dishwasher pump (fig. 26) made from
Hostacom G3 N01 is to be detachably fastened to the
pump housing with eight sheet metal screws. O-ringseal; average diameter of sealing groove dN = 140 mm;
delivery pressure of pump 0.7 bar = 0.07 N/mm2; maxi
mum operating temperature 80C; average housing wallthickness 2.5 mm. What would be a suitable screw size?
Fig. 26: Dishwasher pump (diagram)
*tf_
-IF
For trial purposes, sheet metal screws no. 8 (nominaldiameter d = 4.2 mm) are chosen. A boss outside dia
meter D of 11 mm, receiving hole diameter dK of 3.6 mm
(in accordance with table 2) and screw engagementlength L of 10 mm are chosen. The force acting on the
screw is compared with that permitted over the longterm in order to decide on its suitability.
Force FI resulting from the delivery pressure:It is assumed for safety's sake that the delivery pressureacts on the entire pump top surface. The surface area is
A = ^-- (140 mm)2 = 15400 mm2
With p = 0.07 N/mm2, the force
F, = p A
= 0.07 N/mm2 15400 mm2
F! = 1080 N
Force F2 resulting from deformation of the O-ring;The groove depth is selected to be 80 % of O-ring thick
ness in line with the recommendations of the O-ringmanufacturer. To compress the O-ring by the requiredamount, a force of 1.5 N per mm length is required. The
length of the O-ring is
L = jt- 140 mm
L = 440 mm
Thus the force F2 = 440 mm 1.5 N/mm
F2 = 660 N
The total force to be taken by the eight screws is
Ftotal = F2 + F2= 1080 N + 660 N
Ftotal = 1740 N
Each screw has to take a force of ;; = 217 N.
Permissible long-term screw load Fperm.: Given that the
outside diameter of the boss D = 11 mm, the ratio
I)d
11
4.2= 2.6 > 2.5 (section 4.1.3),
i.e. in the event of an overload, the internal thread will
shear. The appropriate material characteristic value
according to table 3 at 80C is
Ks = 19 N/mm2.
The area under shear stress is
A, = d Jt L
= 4.2 mm n 10 mm
AI = 132 mm2.
Thus the pull-out force
F = A! Ks= 132 mm2 19 N/mm2
F = 2508 N
If we permit 25 % of this value as the maximum con
tinuous load (see section 6.2)
Fperm. = 0.25 2508 N
Fperm. = 627 N
then the actual load (217 N) is substantially smaller than
the permissible.
17
Example 2 Example 3
A car sun roof slideway made from Hostaform C 9021 is A car tailgate handle made from Hostaform C 9021 isto be fastened with thread-forming screws. At one point, fixed with M 6 screws which are driven into moulded-inthere is not enough space to provide a boss - the screw threaded inserts as specified in DIN 16903. What shouldmust be driven directly into the 6 mm wall (L = 6 mm). the driving torque be?What would be the pull-out force of a no. 10 screw (d =4.8 mm) at a temperature of 80C? According to fig. 18, the stripping torque of a moulded-
in M 6 threaded insert is about 10 N m. According to
When overloaded, the joint will fail by shearing of the section 6.1, the permissible driving torque for Hostaforminternal thread so that the load characteristic is about 25% of this value. Thus the driving torque
should be
Kg = 28 N/mm2
MA = 0.25 10 N m
from table 3 applies. MA = 2.5 N m
For the area calculated from the screw diameter d andscrew engagement length L
AI = it d L= n- 4.8 mm 6 mm
A] = 90 mm2,
the pull-out force
F = A Ks= 90 mm2 28 N/mm2
F = 2520 N
18
10. Applications
Screwed joint between the housing and top of a head
lamp washer unit made from Hostaform C 9021 withsheet metal screws BZ no. 4 x 9.5 DIN 7972 (cuttingedge in the first thread turns).
Screwed joint between the pump housing and top of a
dishwasher pump made from Hostacom G3 N01 with
EJOT-PT screws 5 x 14.
( M8-M
knurl
Cooling water filter made from Hostaform C 9021
with threaded inserts M 8 x 12 installed by heated tool
for the screwed joint between the filter housing andthe engine block of a ship's engine.
19
siF\
Screwed joint on a truck heater housing made fromHostacom M4 N01 with quick-fix nut and sheet metalscrew B No. 10 x 19.
Handle plate made from Hostaform C 9021 withffioulded-jn threaded bolts M 5 x 10 for the screwedjoint between the handle plate and car doer.
Itmi
20
11. Explanation ofsymbols 12. Literature
Symbol Unit Explanation
A!
Kz
TB
mmz
mirr
d
di
d2
dB
dK
D
Er
F
i^perm.
h
Ks
mm
mm
mm
mm
mm
mm
N/mm2
N
N
mm
N/mm2
N/mm2
cylindrical area in moulded bosses
(fig. 14a)
circular area in moulded bosses
(fig. 14b)
nominal screw diameter
outside diameter of screw
root diameter of screw
outside diameter of threaded insert
(threaded bolt)
receiving hole diameter
outside diameter of mouldedbosses
relaxation modulus of the plastic
pull-out force, failure load
permissible axial force in
the screw
thread depthshear strength of the plasticin moulded bosses (table 3)
tensile strength of the plasticin moulded bosses (table 3)
L
M
MA
P
P
t
X
mm
N-m
N-m
N/mm2
mm
mm
mm
screw engagement length
stripping torque (fig. 17, 18)
driving torque
compressive stress,
delivery pressure
thread pitchthread bite
receiving hole interference fultrasonic installation of threaded
inserts
thread angle
N/mm2 shear strength between the plasticpart and threaded insert (table 4)
proportional to
approximately equal to
H. Schmidt, H. Röber: Verbinden von Kunststoff-formteilen durch Metallschrauben, VDI-Z, No. 13,1972, p. 967
H. Schmidt: Form- und kraftschlüssige Verbindung vonausgewählten Baugruppen, Industrie-Anzeiger, No. 95,Issue Kunststoffe - Maschinen, Verarbeitung,Anwendung" (No. 11), 15. 11. 1974
H. Großberndt, K. Ociepka: Selbstformende Schraubenfür Thermoplaste - Gewindeprofile und Auslegen der
Einschraubtuben, Kunststoffe, No. 6, 1979, p. 344
DIN 13 Metric ISO thread
DIN 7970 Thread and screw ends for sheet metal
screws
DIN 7998 Thread and screw ends for wood screws
DIN 16903 Threaded bushings for plastic mouldings
21
Engineering plasticsDesign Calculations Applications
Publications so far in this series:
A. Engineering plasticsA. 1.1 Grades and properties - HostaformA. 1.2 Grades and properties - HostacomA. 1.4 Grades and properties - Hostalen GURA. 1 .5 Grades and properties - Celanex,
Vandar, ImpetA.2. 1 Calculation principlesA.2.2 Hostaform - Characteristic values and
calculation examplesA.2.3 Hostacom - Characteristic values and
calculation examples
B. Design of technical mouldingsB. l
.1 Spur gears with gearwheels made from
Hostaform, Celanex and Hostalen GURB.2.2 Worm gears with worm wheels made from
HostaformB.3.1 Design calculations for snap-fit joints in
plastic partsB.3.2 Fastening with metal screws
B.3.3 Plastic parts with integrally moulded threadsB.3.4 Design calculations for press-fit jointsB.3.5 Integral hinges in engineering plasticsB.3.7 Ultrasonic welding and assembly of
engineering plastics
C. Production of technical mouldingsC.2.1 Hot runner system - Indirectly heated,
thermally conductive torpedoC.2.2 Hot runner system - Indirectly heated,
thermally conductive torpedoDesign principles and examples of mouldsfor processing Hostaform
C.3.1 Machining HostaformC.3.3 Design of mouldings made from
engineering plasticsC.3.4 Guidelines for the design of mouldings
in engineering plasticsC.3.5 Outsert moulding with Hostaform
22
In this technical information brochure, Hoechst aims to
provide useful information for designers who want to
exploit the properties of technical plastics such as Hosta-form. In addition, our staff will be glad to advise you on
materials, design and processing.
This information is based on our present state of knowl
edge and is intended to provide general notes on our
products and their uses. It should not therefore be con
strued as guaranteeing specific properties of the productsdescribed or their suitability for a particular application.Any existing industrial property rights must be observed.The quality of our products is guaranteed under our
General Conditions of Sale.
Applications involving the use of Hostaform andHostacom are developments or products of the plasticsprocessing industry. Hoechst as suppliers of the startingmaterial will be pleased to give the names of processorsof plastics for technical applications.
© Copyright by Hoechst Aktiengesellschaft
Issued in August 1996/3 rd edition
23
Hostaform®, Celcon®
polyoxymethylene copolymer (POM)
Celanex®
thermoplastic polyester (PBT)
Impet®
thermoplastic polyester (PET)
Vandar® thermoplastic polyester alloys
Riteflex®
thermoplastic polyester elastomer (TPE-E)
Vectra®
liquid crystal polymer (LCP)
Fortron®
polyphenylene sulfide (PPS)
Celstran®, Compel® long fiber reinforced thermoplastics (LFRT)
GUR®
ultra-high molecular weight polyethylene (PE-UHMW)
EuropeTicona GmbHInformation ServiceTel.: +49 (0) 180-5 84 26 62 (Germany) +49 (0) 69-30 51 62 99 (Europe)Fax: +49 (0) 180-2 02 12 02eMail: [email protected]: www.ticona.com
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