The principal distinction is between detachable and non
detachable joints. A joint is regarded as detachable if the parts can be taken
apart without destroying the connecting elements: for example, screw joints,
pin joints, key joints and press-fit joints. A joint is non detachable, on the
other hand, it it can only be separated by destroying the connecting elements,
for example glued joints, riveted joints, soldered joints or welded joints.
1. Screw joints
Screws are mechanical elements
with a wide range of uses. They are distinguished mainly by the shape ofthe
screw head and the material from which they are produced.
10.1.1 Screws and nuts
Hexagonal head screws and bolts
are used with nuts in through holes and without nuts in threaded holes, in
which an internal thread is cut in the actual workpiece.
Studs are used if the joint is to be separated frequently, The shorter threaded
end of the stud is screwed into the workpiece and tightened with a stud setter.
When the threaded joint is taken apart, the stud remains in the workpiece.
Hexagon socket head
screws save space as a consequence of their cylindrical
heads, which can also be countersunk. There is a special pattern of hexagon
socket head screws with internal serrations. Hexagon socket head screws are
suitable for screwed joints which cannot, because of their position, be
tightened with an ordinary open ended wrench or ring spanner.
Body-fit bolts are used if relative movement between the workpleces must be
prevented, and shear forces are high. Their shank diameter is slightly greater
than the thread diameter. Anti fatigue bolts are used in screwed
joints subject to continuous alternating loads, for example connecting rod big
ends. lf standard bolts are subjected to continuous alternating loads in this
way, they break as a result of fatigue after a period of operation, even if
they are nominally of sufficient strength. In such cases, anti-fatigue bolts
(neck bolts) have a much longer working life, Their shank diameter is only
about 90% ofthe minor diameter of the screw thread except at the points where
they contact the hole. The anti fatigue bolt is tightened with a torque wrench
to the value specified by the manufacturer. This preloads it with a tensile
force significantly greater than that which acts on it externally during
operation. The anti-fatigue bolt can be stressed in the elastic range during
operation until close to its elastic limit. Anti-fatigue bolts tightened to the
specified torque retain their preload and need no additional locking devices.
Their threads and those of the corresponding nuts must turn freely during
assembly of the joint.
Slotted head screws
and cross-recessed (Phillips head) screws can have various
head patterns: half round (domed head),
cheesehead, countersunk, raised cheesehead or raised countersunk.
Grub screws are screws without a head, threaded for their entire length.
Depending on the application they can have various ends: coned point, full dog
point, cupped point. Grub screws (setscrews) are used for the clamping or
locking of hubs. bushes or bearings.
Wood screws are available with slotted, square or hexagonal heads. They are
used for joints between wooden parts.
Sheet metal screws
(self-tapping screws) are used to make joints with sheet
metal. Their threads are similar to wood screws. As they are tightened, they cut
their own thread in the sheet metal. The hole in the sheet should have the same
diameter as the core of the screw. Clamp nuts (sheet metal nuts) are also
frequently used. The joints are vibration resistant and can be separated any number
of times. Sheet metal screws are available with slotted, cross recessed or hexagonal
heads.
Self-tapping screws are surface-hardened screws. They cut their own threads as they
are inseited into the core hole. They are tapered to make them easier to start.
Since considerable force is required to tighten them, self tapping screws are
produced only in sizes M 2.5 to M 8. They are suitable for all materials up to
a tensile strength of 600 N/mm?. They should be dipped in oil before screwing
in. When tightening, pressure must be applied along the axis of the screw.
Thread inserts and
thread bushes are used if the thread is to be cut in a soft
material, if the screwed joint has to be detached often or il the thread
already cut in the workpiece has been
damaged. Thread inserts consist ot a rhomboid-form chrome
nickel steel wire formed into a coil to produce an inner and an outer thread. A
screw thread is cut in the core hole with a special tap. An assembly tool is used
to insert and preload the thread insert. Thread bushings cut their own threads
when inserted.
Nuts are also manufactured in various forms. Hexagonal nuts have a thickness
otabout0.8 x d, or about0.5 x d in the case of special thin nuts.
Castle (castellated)
nuts with 6 or 10 slots are used if a split pin is to be inserted as a keeper.
Cap nuts cover
the screwed joint from the outside to protect the thread against damage, avoid
injury and achieve a smarter appearance.
Union nuts are
used for pipe joints.
Wingnuts and knurled
nuts can be tightened by hand, without tools.
Slotted round and capstan
nuts with metric fine pitch thread are mostly used to lock ball and roller
bearings on to shafts and to adjust endplay.
Weld nuts are
used in coach building. The weld nut is spot welded electrically to the
component at three raised areas. A ring shaped collar centres the nut in the
hole.
Property classes for
screws and nuts
Steel screws are marked with the manufacturer’s symbol and
the strength rating. This consists of two numerals separated by a point. The first number is one-hundredth of the minimum tensile
strength in N/mm2, in our example a minimum tensile strength of 1 200 N/mm2.
The second number is ten times the apparent yield point ratio (minimum apparent
yield point in relation tominimum tensile strength). The product of the two numbers,
e.g. 12 ><9 = 108, corresponds to 0ne tenth of the minimum elastic limit
ol 1080 N/mm?.
Steel nuts are
marked with the manufacturers symbol and a numeral which is one-tenth of the
proof stress in N/mm2, for example 10 means that the nut can be loaded to a
proof stress of 1000 N/mm2, The proof stress of the nut corresponds to the
minimum tensile strength of the
corresponding screw. lf these screws are loaded up to the
minimum ultimate load, then a nut must be chosen with a proof stress value at least
as high as the minimum tensile strength of the screw,
10.1.2 Screw locking
devices
The threads of screws used for clamping purposes are
self-locking, but vibration can cause the nut or the screw to work loose,
Locking devices are intended to prevent this.
Spring washers
and toothed or serrated lock washers apply a load to the nut and also bite into
its surface and that of the workpiece.
Convex or corrugated spring washers are also used. In the
case of double nuts (locknuts,
counter-nuts or check nuts), the top nut elongates the end of the screw or bolt
within the elastic range as it is tightened down against the lower nut, This
presses the two nuts together and prevents them from coming loose.
Spring action
locknuts are hexagon nuts stamped from sheet metal. They are tightened
against the main nut like a normal counter nut. As pressure builds up, their
spring teeth press against the flanks of the bolt thread and prevent the main
nut from loosening.
Lock washers are
made from thin steel sheet. lt a wire keeper is specified, a soft steel wire is
inserted through holes in the screw head and twisted to tighten it. The wire
must be attached in such a way that it is stressed in tension if a screw tends
to work loose.
Castle nuts can
be locked very reliably by means of split pins, However, the range oi
adjustment is limited by the number of slots on the nut.
Elastic stop nuts
(self-locking nuts) have a firmly located polyamide ring at the end of the
thread, which is pressed into the bolt threads when the nut is screwed on, and prevents
the screwed joint from working loose. The friction exerted by the polyamide
ring means that greater tightening torque is required. These nuts must be used
only once.
Washers improve
the nut contact surface and reduce the danger of the nut seizing when
tightening or loosening. inclined or spherical washers can be used to
compensate for contact faces which are not parallel.
10.1.3 Screw tightening
tools
Screwdrivers with a plain blade are used for slotted head
screws. To prevent damage to the screw head, the screwdriver blade must be of the
correct width and thickness. It must be flat and not ground to a wedge shape. For the various types of cross recesses screws there are
special patterns of screwdriver.
The leverage exerted by a spanner or wrench should be sufficient to obtain an adequately firm screwed joint after tightening,
with no risk of overstressing or stripping the thread. This can occur if the
spanner is extended by attaching a length of tube to its handle, in which case
the thread may well be damaged beyond repair.
Spanners should always be an exact fit on the bolt head or
nut. lf the spanner is too large, the bolt head or nut will be damaged and will
tend to become round. so that even the correct size of spanner no longer fits;
furthermore, a slipping spanner could cause an accident.
Torque wrenches indicate the amount of torque being exerted on a suitably
calibrated scale. If the values specified by the automobile manufacturer, for
instance, are adhered to, all threaded connections on the vehicle will be
tightened correctly.
Screws and nuts should not be tightened with too much force
or they may be damaged; if they are too loose, on the other hand, they could fail
to grip and work loose.
Most torque wrenches can be set to a definite value. lf this
is exceeded, the wrench slips and therefore cannot overtighten the joint. There
are many plug-in heads and inserts for torque wrenches, particularly for
tightening various sizes of hex bolts and nuts.
Home
