14-05-2013, 03:28 PM
STUDY OF SCREW THREADS, KEYS AND FASTENERS
Aim:
To study the Screw Threads, Keys and fasteners
SCREW THREADS1.1 INTRODUCTION
A screw thread is a functional element used on bolt, stud, set screw, nut or any other threaded piece or component. Screw thread is a helical groove on a cylinder surface (outer side or inner side). Its function is to transform the input motion of rotation into output motion of translation. If a cylindrical rod is rotated at a constant speed simultaneously if a pointed tool touching the rod moving parallel to the axis of the rod at constant speed, the cut made by tool on the rod will be continuous and of helical form. The helical groove is called “thread” and the threaded rod is called a “screw”. Threads are cut using a lathe. Small size thread is often cut by means of a tool called die. A small size hole is threaded by means of a tool called a tap. The principal uses of threads are,
1.For fastening
2.For adjusting
3.For transmitting power
Lead:
It is the distance measured parallel to the axis from a point on a thread to the corresponding point on the same thread for one complete revolution. In other words, it is axial distance a screw advances in one revolution. When all the threads on a member are built on a single helix it is termed as “single start thread”. It has only one starting point. For a single start thread, the lead and pitch are same. If a quick acting thread is needed, the lead must be large. If a single start is cut with large pitch, the thread depth will also be more, so the amount of material removed is more and the thread will be weakened. To avoid this multiple threads are used when a quick advance is required in a screw pair. Two or more threads are cut side by side around the cylinder on an equal number of parallel helices. It is termed as “Multi start thread”.
Lead = number of starts x pitch A common example for multi start thread is the thread on a fountain pen cap.
DIFFERENT THREAD PROFILES
Threads are standardized to permit to interchangeability of bolts and nuts of the same nominal diameter. The profile of a screw thread is based on its use. The two main kinds are „V‟ and square with various modifications. When the thread has a „V‟ cross-section it is called a V‟ thread and when it has square cross section it is called a square thread.
SPLIT PIN LOCKING USING SLOTTED NUT:
In this method a slotted nut is used instead of the ordinary nut. The slotted nut has slots cut through the opposite parallel hexagonal faces. When the bolt is fully tightened, one set of slots on the opposite faces will come in line with the hole in the bolt. The split pin is inserted through the slots in the nut and the hole in the bolt and then open ends of the split pin are opened.
SPLIT PIN LOCKING USING CASTLE NUT:
In the slotted nuts, the number of effective threads is reduced due to the slot which reduces the strength of the nut considerably. Therefore instead of cutting the slots within the effective depth, they are cut in the extra cylindrical projection provided at the top of the nut. This type of nut is called castle nut. The castle nuts are use don the wheel shafts of automobiles. In the reassembly of the slotted or castle nut, the slots may not come in the alignment with the hole in the bolt. In such cases, the nut is removed from the bolt and its lower face is filled until it can be screwed tightly so that one set of slots come in alignment with the hole in the bolt.
SET SCREW LOCKING USING GROOVED NUT:
A hexagonal nut provided with a cylindrical grooved collar at its lower end is called ring or grooved nut. The end of the bolt hole is counter bored to receive the cylindrical lower grooved portion of the nut. Locking of the nut is done by a setscrew screwed through the nearest face of the work piece. The projection dog-end of the set-screw enters the groove in the cylindrical portion of the nut and prevents the slacking of the
nut. This method of locking is possible if the bolt hole is close to the nearest vertical edge as in the case of marine engine connecting rods. When the bolt hole is not close to the vertical edge of the work piece, this nut is used in conjunction with separate collar. The dowel pin screwed in the bearing surface prevents the rotation of the collar.
RESULTS:
Thus the studies of Screw threads, keys and fasteners were studied.