Kinds of screws

ACKNOWLEDGEMENT

I take this opportunity to present my votes of thanks to all those guidepost who really acted as lightening pillars to enlighten our way throughout this project that has led to successful and satisfactory completion of this study.

We are really thankful to "Miss. Manoj kumar" for her active support, valuable time and advice, whole-hearted guidance, sincere cooperation and pains-taking involvement during the study and in completing the assignment of preparing the said project within the time stipulated.

Lastly, We are thankful to all those, particularly the various friends , who have been instrumental in creating proper, healthy and conductive environment and including new and fresh innovative ideas for us during the project, their help, it would have been extremely difficult for us to prepare the project in a time bound framework.

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crew jack

a) Screw Jack:-

All metallic constructions and a accurately machine cut screw with a pitch of 5 mm crrying a double flanged turn table of about 20 cm dia fitted on a heavy cast iron base and complete with two adjustable pulleys cord & hooks without wts.

Small size, experimental demonstration type model with an aluminum turned pulley of about 10 cm dia is fitted on a screw jack which is fitted on a 12cm dia metallic circular base with an adjustable pulley and a linear vertical scale, over all height is about 15 cms with out wts.

A screw is a shaft with a helical groove formed on its surface. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force.

Threaded Fastener

A screw used as a threaded fastener consists of a shaft, which may be cylindrical or conical, and a head. The shaft has a helical ridge or thread formed on it. The thread mates with a complementary helix in the material. The material may be manufactured with the mating helix, or the screw may create it when first "driven" in. The head is specially shaped to allow a screwdriver to grip the screw when driving it in. It also stops the screw from passing right through the material being fastened and provides compression.

Screws can normally be removed and re-inserted without reducing their effectiveness. This may make them preferable in some applications to nails, which are frequently unusable after being removed.

A screw that is tightened by turning it clockwise is said to have a right-hand thread. Screws with left-hand threads are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a right-hand thread.

Bolt

Mechanical Analysis

A screw is a specialized application of the wedge or inclined plane. It contains a wedge, wound around an interior cylinder or shaft that either fits into a corresponding plane in a nut, or forms a corresponding plane in the wood or metal as it is inserted. The technical analysis (see also static's, dynamics) to determine the pitch, thread shape or cross section, coefficient of friction (static and dynamic), and holding power of the screw is very similar to that performed to predict wedge behavior. Wedges are discussed in the article on simple machines.

Tensile Strength

Screws and bolts are usually in tension when properly fitted. In most applications they are not designed to bear large shear forces. When, for example, two overlapping metal bars joined by a bolt are likely to be pulled apart longitudinally, the bolt must be tight enough that the friction between the two bars can overcome the longitudinal force. If the bars slip then the bolt may be sheared in half, or friction between the bars (called fretting) may weaken them. For this type of application, high-tensile steel bolts are used and these should be tightened with a torque wrench.

High-tensile bolts are usually in the form of hexagonal cap screws with an ISO strength rating stamped on the head. The strength ratings most often used are 8.8 and 12.9. The number before the point is the ultimate tensile strength in N/mm2 (or MPa) divided by 100. This is the stress at which the bolt will fail, i.e. break in half.

The number after the point is the yield strength as a percentage of the ultimate tensile strength, divided by 10. Yield strength is the stress at which the bolt will receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2%.

Mild steel bolts have a 4.6 rating. High-tensile bolts have an 8.8 rating or above.

Types of screw jack

Cap screw

: -

Wood screw

: -

Machine screw

: -

Self-tapping screw

Drywall screw

Set screw

Dowel

screw

Shapes of Screw Head

Pan

Head

:

Button

or

dome head

Round

Truss

Flat

or

Countersunk

Oval

Cheese head

Mirror screw head

Types of

Screw Drive

Modern screws employ a wide variety of drive designs, each requiring a different kind of tool to drive in or extract them. The most common screw drives are the slotted and Phillips; hex, Robertson, and torx are also common in some applications. More exotic screw drive types may be used in situations where tampering is undesirable, such as in electronic appliances that should not be serviced by the home repairperson.

Slot

Head has a single slot, and is driven by a flat-bladed screwdriver. The slotted screw is common in woodworking applications, but is not often seen in applications where a power driver would be used, due to the tendency of a power driver to slip out of the head and potentially damage the surrounding material.

Cross-head

or

Phillips

Screw has a "+"-shaped slot and is driven by a cross-head screwdriver, designed originally for use with mechanical screwing machines. The Phillips screw drive has slightly rounded corners in the tool recess, and was designed so the driver will slip out, or cam out, under strain to prevent over-tightening.

  Pozidriv

           it   is patented, similar to cross-head but designed not to slip, or cam out. It has four   additional points of contact, and does not have the rounded corners that the Phillips screw drive has. Phillips screwdrivers will usually work in Pozidriv screws, but Pozidriv screwdrivers are likely to slip or tear out the screw head when used in Phillips screws. Pozidriv was jointly patented by the Phillips Screw Company and American Screw Company.

Torx

:

-

Hexagonal or hex

:-

Robertson

Tri-Wing

Torq-Set

Spanner

:

-

It is said that a chain is as h3 as its weakest link, so is true for a structure. For a structure or a truss to sustain load all its members should be able to stand force acting on them. For optimal design of a structure it is essential to know the forces in the members, compressible or tensile.

Structures are made to support loads. Structure is an assembly of number of members arranged in certain manner. When load acts on a structure this load is distributed to the constituent members of the structure in different proportions. Members experiencing large forces can be made stronger, members experiencing less force can be made lighter and redundant members with no force to support can be removed altogether. Thus static force analysis of structures can help to build cost effective, light and h3 structures.

Static Force Analysis in Structures

Force acting on a member of a structure can be compressible or tensile. For the purpose of force analysis a sign convention can be assigned to the forces. Tensile forces, acting outwards the members and having a tendency to extend the member, is assigned positive sign. The force acting inwards any member and tending to compress the member is called as compressive force and assigned negative sign. Although a sign convention opposite to this one can also be followed with equal validity but the same sign convention should be adhered to throughout the analysis of a structure.

There are mainly two approaches for static force analysis in structures. One approach is to section the structure under consideration and find the unknown forces by balancing the forces. Other approach is based on the principle that net force at any joint or node for static structure is zero. In either of the approaches force calculation is started from the support points as it is easier to determine the forces at the support points and further calculation of forces in the members of the structure becomes easier after knowing the forces at supports.

Section Approach

In the Section Approach the structure under consideration is sectioned at certain part such that the number of unknown forces is not more than two, for two dimensional structures. Unknown forces are assigned variables and components of the forces are taken along and perpendicular to any one of the unknown forces. For each of the two directions force balance equations are framed and solved for the unknowns. The components of the forces can also be taken along any fixed coordinate axis. For three dimensional structures the section taken can have up to three unknown forces.

Nodal Approach

In the other approach, to find forces in the members of a structure, net force at any joint is set to zero. Any joint connecting two or more members can be called as a node. One by one different nodes are considered for force analysis. To start with such a node will be taken which has not more than two unknown forces. Unknown forces are determined by writing net forces along any set of orthogonal axes and equating them to zero.

For a simple structure one of the two approaches may be sufficient to determine the forces in members. But for complex structures single force analysis approach can become cumbersome for force analysis, therefore, a tricky combination and use of the two approaches can simplify the static force analysis in structures.

REFERENCES

• Basics of Engineering Mechanics: Introduction
• Basics of Engineering Mechanics: Statics
• Engineering Mechanics: Force Analysis in Statics
• 4.Engineering Mechanics: Contact Forces
• www.google.com
• 6www.wikipedia.com
• www.physcience.com

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