Joining Techniques

Joining techniques

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.