Mini Project Screw Jack 2 PDF

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CHAPTER 1 INTRODUCTION Engineers play a key role in the development of our society, contributing towards building the economy and inspiring changes that improve on the quality of life. They possess the ability to comprehend technological processes and creative thinking skills which can help in the solving of the present problems in both business and the industrial world. Due to global and technological changes in the world today there is a need for research and development activities to help counter this, and this can be in terms of complete or slight changes from the existing technology and all this work requires an engineer. In an effort to improve the quality of life a power screw was invented, which is also called a translational screw that converts rotary motion into translation motion. Power screws have many applications such as i.

To raise the load, e.g., screw jack;

ii.

To obtain accurate motion in machining operations, e.g., lead screw of lathe;

iii.

To clamp a work piece, e.g., a vice; and

iv.

To load a specimen, e.g., universal testing machine.

The screw jack is one of the power screws in which a small force is required to be applied to raise or lower a large load. Definition: A screw jack is a portable device consisting of a screw mechanism used to raise or lower the load in order to facilitate vehicle maintenances or breakdown repairs. A screw jack consists of a heavy-duty vertical screw with a load table mounted on its top, which screws into a threaded hole in a stationary support frame with a wide base resting on the ground. A rotating collar on the head of the screw has holes into which the handle, a metal bar, fits. When the handle is turned clockwise, the screw moves further out of the base, lifting the load resting on the load table. In order to support large load forces, the screw usually has either square threads or buttress threads.There are two types of jacks – hydraulic and mechanical. The movement of the piston rod is used to raise or lower the load. In normal Jack system a mechanical jack (hand operated or power driven) is used for lifting the vehicles. The most common form is a car jack, garage jack, floor jack which lifts vehicles so that maintenance can be performed. Car jacks generally used to increase mechanical advantage while lifting the vehicle. In general the weight of the vehicle is near about the 1 tons.

A

specified jack can hold up to 1000 kilograms, but tests taken by Consumer Affairs has revealed

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that is fails to work after lifting 250 kilograms and may physically break when it has a weight close to its 1000 kilograms capacity. Tests have proven that the jack has the tendency to buckle under the weight it is promoted to withstand. Since the use of any lifting device is subjected to certain hazards. In screw jack applications, the hazards are dropping, tipping or slipping of machines or their parts during the operation. Hence, proper size, strength and stability are the essential requirements for the design of the screw jack from safety considerations. 1.1 Parts of Screw Jack Screw Jack is a combination of seven essential parts. They are – a.) Frame b.) Screw c.) Nut d.) Handle e.) Cup f.) Set Screw g.) Washer

Fig. 1.1 Assembled Screw Jack

Frame – It is conical in shape and hollow internally to accommodate a nut & screw assembly. The complex shape of frame is manufactured by casting process. A FG200 (Grey Cast Iron with ultimate tensile strength of 200N/mm2) as material for frame such as it contains carbon precipitates as “graphite flakes” as graphite is soft in nature which improves its ability to resist a compressive load. Fig. 1.2 Frame Screw - Screw is nothing but a member having Helical groove around periphery of solid bar. Square threads usually turned on lathes using single point cutting tool. It leads us to use free cutting steel.

30C8

indicates 0.3% carbon gives it sufficient strength to compensate weakness in roots and 0.8% manganese makes easy in cutting.

Fig.1.3 Screw

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Nut - As we know there always a relative motion between screw and nut, which results in friction. The friction causes wear for screw & nut. So one out of two has to be softer than other so as to overcome friction. The size & shape of screw is costlier than nut, so generally we use softer material for nut than screw. Phosphor bronze (copper alloy having 0.2%phospher) is ideal material for nut which increases tensile strength. The Advantages of phosphor bronze are good corrosion resistance, low coefficient of friction and higher tensile strength than copper brass.

Fig. 1.4 Nut

Handle - Handle is long circular bar like structure, used to rotate nut. Technically it is called as “Tommy Bar”. It is subjected torsional bending moment. So plain carbon steel with 0.3% carbon i.e. 30C8 is used to manufacture. CUP - It is used to hold object or part. Shape of cup is again complex and so economical to manufacture by casting process, hence material will be cast iron with grade FG200.

Fig. 1.5 Handle

Fig. 1.6 Cup

Fig. 1.7 Setscrew

Fig. 1.8 Washer

Setscrew – The purpose of set screw is to resist motion of nut with screw. It can be of commercial steel. Washer – Washer is to provide uniform force of tightening nut over screw force by enlarging area under actions of force. We can use commercial steel. 1.1.1 Advantages and Disadvantages of the Screw Jack Advantages The load can be kept in lifted position since the screw jack is self-locking. This means it remains motionless where it was left when the rotational force on the screw is withdrawn. It will not rotate backwards regardless of size of the weight. Screw jacks also lift or raise the moderate heavy weights against gravity and uses very small handle force that can be applied manually. They are simple to design and are cheap and affordable.

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Disadvantages The major disadvantage of the screw jack is that chances of dropping, tipping or slipping of the load are high and can cause serious accidents hence the device is termed as not safe fail. They should always be lubricated. Accidents caused by screw jack are due to the following reasons: (a) Improper securing of load on the jack. (b) Overloading. (c) Off centre of axis of the jack with respect to centre of gravity hence not ideal for side loads. (d) Placing the jack on a soft ground and unlevelled surface. (e) Using the jack for wrong purpose instead of using it for the purpose for which it is designed.

Fig.1.9 Screw Jack

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CHAPTER 2 LITERATURE SURVEY 

Lokhande Tarachand et al. [1] optimises the efficiency of square threaded mechanical screw jack by varying different helix angle.



Manoj Patil et al. [2] described in this general article, screw jack is too developed to overcome the human effort. It is actually difficult job to operate for pregnant women and old person. Changing the tyre is not a pleasant experience. Especially women can’t apply more force to operate. For that, electric operated car jack is introduced.



Thomas J. Prather et al [3] introduce about vehicle lift system. A drive assembly was mechanically coupled to the piston. The drive assembly was operated in first direction to raise an upper end of the piston with respect to the housing. The drive assembly was operated in a second direction to lower the upper end of the piston with respect to the housing. The drive assembly was coupled to the power supply port which is removable to supply electrical power to the drive assembly.



Farhad Razzaghi et al. [4] designed electrically powered jack for normally raising and lowering of automobile from ground surface. The mechanism may be used in joining with a typical portable car jack, during which the mechanism constitutes a power drill, a rod, and a numerous jack adapters.

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CHAPTER 3 CLASSIFICATION OF SCREW THREADS 3.1 Introduction The threads used for fastening purposes, such as V threads are not suitable for screw jack. The purpose of fastening threads is to provide high frictional force, which lessons the possibility of loosening the parts assembled by threaded joint. On the other hand, the purpose of power transmission threads is to reduce friction between the screw and nut. Therefore, V threads are not suitable for screw jack. Screws with smaller angle of thread are preferred for power transmission. 3.1.1 Square Thread It has a square cross section of the thread. It is the most common form used by the screw jack and used especially in high load applications. Advantages of Square Thread

Fig.3.1 Square thread

1.) The efficiency of square threads is more than that of trapezoidal thread. 2.) They have lower friction coefficient hence less power loss in lifting the load. 3.) There is no radial pressure or side thrust on the nut. This radial pressure is called ‘bursting’ pressure on the nut. Since there is no side thrust, the motion of the nut is uniform. The life of the nut is also increased. Disadvantages of Square Thread 1.) Square threads are difficult to manufacture. They are usually turned on a lathe with a single-point cutting tool. Machining with a single point cutting tool is an expensive operation compared with machining with a multipoint cutting tool. 2.) The strength of a screw depends upon the thread thickness at the core diameter as square threads have less thickness at the core diameter than trapezoidal threads. This reduces the load carrying capacity of the screw. 3.) It is not possible to compensate for wear in square threads since wear of the thread surface becomes a serious problem in the service life of the screw jack. Therefore, replacement of the nut or the screw is required when worn out. Applications: - Square threads are used for screw-jacks and presses.

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Nominal diameter, d

Pitch, p

(mm) 22,24,26,28 30,32,36

(mm) 5 6

40,44 48,50,52

7 8

55,60

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65,70,75,80 10 85,90,95,100 12 Table 3.1 Proportions of square threads (normal series)

3.1.2 ISO Metric Trapezoidal Threads These are threads with trapezoidal outline profile. They are most commonly used for lead screws. They offer high strength and ease of manufacture. Fig.3.2 Trapezoidal thread Advantages of the Trapezoidal Thread 1.) Trapezoidal threads are manufactured on a thread milling machine. It employs a multipoint cutting tool. Machining with a multipoint cutting tool is an economic operation compared with machining with single point cutting tool. Therefore, trapezoidal threads are economical to manufacture. 2.) Therefore, a screw with trapezoidal threads is stronger than an equivalent screw with square threads. Such a screw has large load carrying capacity. 3.) The axial wear on the surface of the trapezoidal threads can be compensated by means of a split-type of nut. The nut is cut into two parts along the diameter. As wear progresses, the looseness is prevented by tightening the two halves of the nut together. The split-type nut can be used only for trapezoidal threads. It is used in lead-screw of lathe to compensate wear at periodic intervals by tightening the two halves. Disadvantages of the Trapezoidal Thread 1.) The efficiency of trapezoidal threads is less than that of square threads. 2.) Trapezoidal threads result in side thrust or radial pressure on the nut. The radial pressure or bursting pressure on the nut affects its performance.

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Application: - Trapezoidal and acme threads are used for lead-screw and other power transmission devices in machine tools. Nominal diameter, d

Pitch, p

(mm) 24,28 32,36 40,44

(mm) 5 6 7

48,52

8

60 70,80

9 10

90,100 12 Table 3.2 Proportions of ISO metric trapezoidal threads

Note –: There is a special type of trapezoidal thread called acme thread. Trapezoidal and acme thread are identical in all aspects except the thread angle. The thread angle in acme thread is 29 °

instead of 30 ° . The relative advantages

and

disadvantages of acme threads are same as those of trapezoidal threads.

Fig.3.3

Acme Thread 3.1.3 Buttress Thread Buttress thread is a combination of advantages of square and trapezoidal threads. These threads are used where a heavy axial force acts along the screw axis in one direction only. Advantages of Buttress Thread 1.) It has higher efficiency compared to trapezoidal

threads.

It

can

be

economically manufactured on a thread milling machine. 2.) The axial wear at the thread surface can be compensated by means of split-type nut. Fig.3.4 Buttress Thread

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3.) A screw with buttress threads is stronger than equivalent screw with either square threads or trapezoidal threads. This is because of greater thickness at the base of the thread. Disadvantages of Buttress Thread 1.) It can transmit power and motion only in one direction as compared to square and ISO metric trapezoidal threads, which can transmit force and motion in both directions. Application: - Buttress threads are used in vices, where force is applied only in one direction.

3.2 Terminology of Screw Jack a.) Pitch - The pitch is defined as the distance measured parallel to the axis of the screw from a point on one thread to the corresponding point on the adjacent thread. It is denoted by the letter p. b.) Lead - The lead is defined as the distance measured parallel to the axis of the screw that the nut will advance in one revolution of the screw. . It is denoted by the letter l. For a single threaded screw � = � For a double threaded screw � = �� c.) Nominal Diameter – Nominal Diameter is the largest diameter of the screw. It is also called major diameter and is denoted by the letter d. d.) Core Diameter – The core diameter is the

Fig.3.5 Terminology of Screw Jack

smallest diameter of the screw thread. It is also called minor diameter and is denoted by the letter dc.

[ ]

d c =d −

p p + ∨d c =(d− p ) 2 2

e.) Mean Diameter – It is denoted by dm. d m=

( d +d c ) 1 = [ d +(d − p )] 2 2 d m= ( d−0.5 p)

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f.) Helix angle – the helix angle is defined as the angle made by the helix of the thread with a plane perpendicular to the axis of the screw. The helix angle is related to the lead and the mean diameter of the screw. It is also called lead angle and is denoted by α . Taking one thread of the screw and unwinding, one complete turn is developed. The thread will become the hypotenuse of a right-angled triangle with the base π d m and height being equal to the lead �. This right-angled triangle gives the relationship between the helix angle, mean diameter and lead, which can be expressed in the following form: tan α=

l π dm

Where

Fig. 3.6 Development of Thread

α is the helix angle of the thread.

The following conclusions can be drawn on the basis of the development of thread: 

The screw can be considered as an inclined plane with � as the angle of inclination.



The load � always acts in the vertical downward direction. When the load � is raised, it moves up the inclined plane. When the load � is lowered, it moves down the inclined plane.



The load � is raised or lowered by means of an imaginary force � acting at the mean radius of the screw. The force � multiplied by the mean radius (��/2) gives the torque � required to raise or lower the load. Force � is perpendicular to load �.

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3.3 Torque Required During Lifting of Load The screw is considered as an inclined plane with inclination � when the load is being raised. The following forces act at a point on this inclined plane: Fig. 3.7 Force Diagram for Lifting Load (i.)

Load (W): It always acts in the vertical downward direction.

(ii.)

Normal reaction (N): It acts perpendicular (normal) to the inclined plane.

(iii.)

Frictional force ( μN ): Frictional force acts opposite to the motion. Since the load is moving up the inclined plane, frictional force acts along the inclined plane in downward direction.

(iv.)

Effort (P): The effort � acts in a direction perpendicular to the load �. It may act towards the right to overcome the friction and raise the load.

Resolving the forces horizontally, P=μN cos α + N sin α

(a)

Resolving the forces vertically, W =N cos α−μN sin α

(b)

Dividing expression (a) by (b), P=

W (μ cos α+sin α) (cos α−μ sin α)

Dividing the numerator and denominator of the right hand side by

P=

W (μ+ tan α) (1−μ tan α)

(c) The coefficient of friction μ can be expressed as follows: μ=tan ϕ

(3.1)

cos α we get

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Where

ϕ is the angle of friction.

Substituting

μ=tan ϕ in equation (c), we have P=

W (tan ϕ+ tan α) (1−tan ϕtan α)

P=W tan (ϕ+α)

Or

(3.2)

The torque Mt required to raise the load is given by: M t=

Pd m 2

M t=

Wd m tan (ϕ+α ) 2

(3.3)

3.4 Torque Requirement During Lowering of Load When the load is being lowered, the following forces act at a point on the inclined plane: (i.)

Load (W): It always acts in the vertical downward direction.

(ii.)

Normal reaction (N): It acts perpendicular (normal) to the inclined plane.

(iii.)

Normal reaction (N): Frictional force acts opposite to the motion. Since the load is moving down the inclined plane, frictional force acts along the inclined plane in the upward direction.

(iv.)

Effort (P): The effort � acts in a direction perpendicular to the load �. It should act towards left to overcome the friction and lower the load.

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Fig. 3.8 Force Diagram for Lowering Load Resolving the forces horizontally, P=μN cos α −N sin α

(a)

Resolving the forces vertically, W =N cos α+ μN sin α

(b)

Dividing expression (a) by (b), P=

W (μ cos α−sin α ) (cos α + μ sin α )

Dividing the numerator and denominator of the right hand side by P= Substituting

W (μ−tan α ) (1+ μ tan α)

(c)

μ=tan ϕ in equation (c), we have P=

Or

cos α we get

W (tan−tan ϕ α) (1+ tantan ϕα )

P=W tan (ϕ−α) M t=

Wd m tan (ϕ−α ) 2

(3.4) (3.5)

3.5 Over Hauling and Self-Locking Screws From equation (3.5), we know torque required to lower load is given by: M t=

Wd m tan (ϕ−α ) 2

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Case 1: When

ϕα tan>ϕ tan α

μ>

l π dm

(3.6)

Therefore, the following conclusions are made: (i.)

Self-locking of the screw is not possible when the coefficient of friction (μ) is low. The coefficient of friction between the surfaces of the screw and the nut is reduced by lubrication. Excessive lubrication may cause the load to descend on its own.

(ii.)

The self-locking property of the screw is lost when the lead is large. The lead increases with number of starts. For double-start thread, lead is twice of the pitch and for triple threaded screw, three times of pitch. Therefore, the single threaded screw is...