The pitch "P": is the distance between adjacent thread forms measured parallel to the thread axis. The lead "l": is the distance that the nut moves parallel to the screw axis when the nut is given one turn. Note that the thread size is specified by giving the pitch p for metric sizes and by giving the number of threads per inch "N".
Note that the letter M preceding the diameter is the clue to the metric designation. The ideal bolt length is one where only one or two threads protrude from the nut after it is tightened. The load is obtained by turning the nut until the bolt has stretched almost to the elastic limit.
If the nut is not loosened, this bolt tension remains as the preload or tightening force. When tightening, the mechanic should, if possible, hold the bolt head still and rotate the nut; this way the bolt shaft will not feel the frictional rotation of the thread.
Joints Fastener
There are 4 basic types of failure mechanisms for bolted connections under shear
Bolt Shear
A larger diameter bolt
Edge Tear-Out
Bearing Failure
This type of failure occurs when one of the plates is too thin or not strong enough for the loads applied.
Net Section Failure
The minimum spacing between the successive bolts should be realized. The spacing is dictated
The bolted connections can be classified based on geometry and loading conditions into three types
1.Shear connections
2.Tension connections
3.Combined shear and tension connections
Shear Connections
The overlapping joint tends to bend so that the forces become collinear. The load transfer at the joint will ultimately be through the shear forces in the bolts. In the case of a lap joint or butt joint with a single cover plate, there is only one shear plane, so the bolts should be in a single shear.
In the case of a double cover butt joint, there are two cutting planes, and then the bolts will be in double displacement. It should be noted that the single cover butt joint is bent so that the center of the cover plate becomes collinear with the forces.
Tension Connection
A bolt (or simply a bolt) is a cylindrical rod "shaft" with a thread for a nut at one end and a head at the other. It is passed through the holes drilled in the two pieces to be joined together and clamped tightly together as the nut is screwed into the threaded end. It is screwed into a tapped hole of one of the parts that will be fixed without a nut.
One end of the pin is screwed into a tapped hole on the parts to be attached. They can be used instead of a key to prevent relative movement between a hub and a shaft in lightweight power transmission elements. D: is the diameter of the shaft (in mm) on which the adjusting screw is pressed.
The following stresses are induced in a bolt, screw or stud when it is tightened.
Tensile stress due to stretching of bolt (Initial Tightening)
When the joint is not required to be as tight as a liquid-tight joint, the initial stress in a bolt can be reduced to half of the above value. Small diameter bolts may fail during tightening, therefore smaller diameter bolts (less than M 16 or M 18) are not permitted when making liquid tight joints.
Torsional shear stress caused by the frictional resistance of the threads during its tightening
- Combined tension and shear stress
Experiments have shown that repeated loosening and tightening of the nut causes a gradual notching of the thread, which increases the torsional moment (T). If the outer surfaces of the parts to be joined are not parallel to each other, the bolt will be subjected to a bending action. From Table 11.1 (rough series) we see that the stress area, i.e. the cross-sectional area at the bottom of the wire, corresponding to M 30, is 561 mm2.
If the load tending to separate these parts is neglected, find the stress placed on the bolt by the initial tightening. Bolts, studs and screws usually carry a load in the direction of the axis of the bolt, which causes a tensile stress in the bolt. dc = Diameter of the root or core of the thread, and. Now from the standard table of threads, the value of the nominal diameter of the bolt corresponding to the value of "dc" or stress area 𝝅 can be obtained.
Sometimes the bolts are used to prevent the relative movement of two or more parts, as in the case of flange coupling, then the shear stress is induced in the bolts. It should be noted that when the bolts are subjected to direct shear loads, they should be located so that the shear load is on the bolt's body (i.e. shank) and not on the threaded portion. In some cases, the bolts can be relieved of shear load by using shear pins.
When a number of bolts are used to share the shear load, the completed bolts must be installed in the holes provided. The diameter of the shaft of the bolt is obtained from the shear load, and. A diameter slightly larger than that required for either shear or tension may be assumed, and stresses due to combined loading should be checked for the following principal stresses.
Determine the nominal diameter of the bolt, if the tensile stress should not exceed 100 MPa. It consists of a circular ring at the head and threaded at the lower portion to screw into a threaded hole at the top of the machine. The coupling flanges are secured with four bolts of the same material with a radius of 30 mm.
Find the size of the bolts if the allowable shear stress for the bolt material is 30 MPa. From the table of standard screws (coarse series), the standard screw core diameter is 3.141 mm in.
APPLICATIONS
Design of cylinder cover plate
The internal pressure within the cylinder tries to lift the cylinder cover, while the bolts or studs try to hold it in place. The point X is the center of pressure for the bolt load and the point Y is the center of internal pressure.
The Mechanics of Power Screws
The load "P" is tension and it causes the connection to stretch or elongate through a distance "δ". This elongation can be related to the stiffness by remembering that k is the force divided by the deflection.
