SCISSOR SCREW JACK 1.0 Introduction Introduction
Jack is a mechanical device used to lift heavy loads or apply great forces. A mechanical jack employ a square thread for lifting heavy equipment. The most common form is a car jack, floor jack or garage jack which lifts vehicles so that maintenance can be performed. Mechanical jacks are usually rated for a maximum lifting capacity. Scissor jacks are mechanical devices and have been in use since 1930s. A scissor jack is a device constructed with a cross-hatch mechanism, much like a scissor, to lift up a vehicle for repair. It typically works in a vertical manner. The jack opens and folds closed, applying pressure to the bottom supports along the crossed pattern to move the lift. When closed, cl osed, they have a diamond shape. Scissor jacks are simple mechanisms used to handle large loads over short distances. The power screw design of a common scissor jack reduces the amount of force required by the user to drive the mechanism. Most scissor jacks are similar in design, consisting of four main members driven by a power screw [7] [9]. A scissor jack is operated simply by turning a small crank that is inserted into one end of the scissor jack. This crank is usually "Z" shaped. The end fits into a ring hole mounted on the end of the screw, which is the object of force on the scissor jack. When this crank is turned, the screw turns, and this raises the jack.
The screw acts like a gear mechanism. It has teeth (the screw thread), which turn and move the two arms, producing work. Just by turning this screw thread, the scissor jack can lift a vehicle that is several thousand pounds. A scissor jack has four main pieces of metal and two base ends. The four metal pieces are all connected at the corners with a bolt that allows the corners to swivel. A screw thread runs across this assembly and through the corners. When opened, the four metal arms contract together, coming together at the middle, raising the jack.
2.0 Objective
To know more about manufacturing process of scissors screw jack
To find out every component in scissors screw jack
To observe every material used to made scissors screw jack
To more understand the theory were learning in the class
3.0 Material Selected
No
Types of Component
Material Used
1
Power Screw
Mild Steel
2
Nut
Bronze
3
Pin in nut
Mild Steel
4
Top Arm
Mild Steel
5
Bottom Arm
Mild Steel
6
Top Plate
Mild Steel
7
Bottom Plate
Mild Steel
8
Trunion
Mild Steel
4.0 Bill of Material
1
3
2
4
6
5
7
Part number
Component Name
1
Top plate
2
Pins
3
Top Arm
4
Trunion
5
Bottom Arm
6
Power Screw
7
Bottom Plate
5.0 Manufacturing Process
Production of Screw Thread The various methods, which are more or less widely employed for producing screw threads are:
5.1.1 Casting Characteristics 1. Only a few threads over short length 2. Less accuracy and poor finish.
5.1.2 Forming (Rolling) Characteristics 1. Blanks of strong ductile metals like steels are rolled between threaded dies 2. Large threads are hot rolled followed by finishing and smaller threads are straight cold rolled to desired finish 3. Cold rolling attributes more strength and toughness to the threaded parts 4. Widely used for mass production of fasteners like screws
5.1.3 Removal process (Machining) 1. Accomplished by various cutting tools in different machine tools like l athes, milling machines, drilling machines (with tapping attachment) etc. 2. Widely used for high accuracy and finish 3. Employed for wide ranges of threads and volume of production; from piece to mass production.
5.1.4 Semi finishing and finishing (Grinding) Characteristics 1. Usually done for finishing (accuracy and surface) after performing by machining or hot rolling but are often employed for direct threading on rods 2. Precision threads on hard or surface hardened components are finished or directly produced by grinding only 3. Employed for wide ranges of type and size of threads and volume of production 5.1.5 Precision forming to near net shape
Characteristics 1. No machining is required, slight grinding is often done, if needed for high accuracy and finish 2. Application – Investment casting for job order or batch production – Injection molding (polymer) for batch or mass production
5.1.6 Non-conventional process (EDM, ECM etc.) Characteristics 1. When conventional methods are not feasible 2. High precision and micro threads are needed 3. Material is as such difficult to process
5.2 Processes, Machines and Tools Used For Producing Screw Threads By (a) Machining (b) Rolling
5.2.1 Production of screw threads by machining
Machining is basically a removal process where jobs of desired size and shape are produced by gradually removing the excess material in the form of chips with the help of sharp cutting edges or tools. Screw threads can be produced by such removal process both manually using taps and dies as well as in machine tools of different t ypes and degree of automation. In respect of process, machine and tool, machining of screw threads are done by several ways:
5.2.1.1 Thread cutting by hand operated tools Usually small threads in few pieces of relatively soft ductile materials, if required, are made manually in fitting, repair or maintenance shops.
(a) External screw threads
Machine screws, bolts and studs are made b y different types of dies which look and apparently behave like nuts but made of hardened tool steel and having sharp internal cutting edges
1. Solid or button die: used for making threads of usually small pitch and diameter in one pass. 2. Spring die: the die ring is provided with a slit, the width of which is adjustable by a screw to enable elastically slight reduction in the bore and thus cut the thread in number of passes with lesser force on hands. 3. Split die: the die is made in two pieces, one fixed and one movable (adjustable) within the cavity of the handle or wrench to enable cut relatively larger threads or fine threads on harder blanks easil y in number of passes, the die pieces can be replaced by another pair for cutting different threads within small range of variation in size and pitch. 4. Pipe die: pipe threads of large diameter but smaller pitch are cut by manually rotating the large wrench (stock) in which the die is fitted through a guide bush as shown in Fig.1.
(a) Solid die
(c) Split die
(b) Spring die
(d) Pipe die
Fig 1 :Different types of thread cutting dies
5.2.1.2 Machining screw threads in machine tools Threads of fasteners in large quantity and precision threads in batches or lots are produced in different machine tools mainly lathes, by various cutting tools made of HSS (high speed steel) or often cemented carbide tools.
(a)Machining screw threads in lathes Screw threads in wide ranges of size, form, precision and volume are produced in lathes ranging from center lathes to single spindle automats. Threads are also produced in special purpose lathes and CNC lathes including turning centers.
In center lathes
External threads
-External threads are produced in center lathes by various methods
Single point and multipoint chasing
As schematically shown in Fig 2 This process is slow but can provide high quality. Multipoint chasing gives more productivity but at the cost of quality to some extent
Fig 2: External threading in lathe by chasing
Thread milling:
This process gives quite fast production by using suitable thread milling cutters in centre lathes as indicated in Fig 6.5. The milling attachment is mounted on the saddle of the lathe. Thread milling is of two types
Figure 3 : Thread milling by attachment in center lathes
Long thread milling
Long and large diameter screws like machine lead screws are reasonably accuratel y made by using a large disc type form milling cutter as shown in Fig 3
Short thread milling
Threads of shorter length and fine pitch are machined at high production rate by using a HSS milling cutter having a number of annular threads with axial grooves cut on it for generating cutting edges. Each job requires only around 1.25 revolution of the blank and very short axial (1.25 pitch) and radial (1.5 pitch) travel of the rotating tool.
6.3 Production of screw threads by thread rolling In production of screw threads, compared to machining thread rolling, • Generally cold working process • Provides higher strength to the threads • Does not cause any material loss • Does not require that high accuracy and finish of the blank • R equires simpler machines and tools • A pplicable for threads of smaller diameter, shorter length and finer pitch • Enables much faster production of small products like screws, bolts, studs etc. • Cannot provide that high accuracy • A pplicable for relatively softer metals • Used mostly for making external screw threads • Needs separate dies for different threads Thread rolling is accomplished by shifting work material by plasti c deformation, instead of cutting or separation, with the help of a pair of dies having same threads desired. Different types of dies and methods are used for thread r olling which include • Thread rolling between two flat dies • Thread rolling between a pair of circular dies • Thread rolling by sector dies
Rolling of external screw threads by flat dies The basic principle is schematically shown in Fig 4. Flat dies; one fixed and the other moving parallel, are used in three configurations
Horizontal: most convenient and common
Vertical: occupies less space and facilitates cleaning and lubrication under gravity
Inclined: derives benefit of both horizontal and vertical feat ures
All the flat dies are made of hardened cold die steel and provided with linear parallel threads like grooves of geometry as that of the desired thread.
Fig 4 Principle of thread rolling by flat dies
6.0 Fabrication
The fabrication process started with identification of suitable materials which are used for prototype and designing of various parts. 6.1 Top Arms and Bottom Arms
7.0 Conclusion
