Transcript
CONTENTS Contents
Sr. No. 1 2
3 4 5
INTRODUCTION 1.1 Necessity / Application of scissor lift 1.2 Literature survey PROPOSED WORK 2.1 Proposed project 2.2 List of components and function 2.3 Working principle
5 6 7 8 8 9 10
2.4 Technical Parameter
12
2.5 Main components and design 2.5.1 Worm and worm Gear
13 13
2.5.2 Rack and Pinion
14
2.5.3 Scissor linkages 2.6 Drafting and 3D model 2.7 Calculations 2.8 Estimated time required
15 16 22 26
2.9 Cost
26
Risk management Advantages and disadvantages Feasibility study
29
5.1 Economic feasibility 5.2 Technical / operational feasibility
6 7
Page No.
Conclusion Future Scope
30 31 31 31 32 33
1
LIST OF TABLES Title
Table No.
Page No.
1.
System Components
10
2.
Technical Parameters
13
3.
Time Required
26
4.
Cost of material
27
5.
Cost of standard parts
27
6.
Miscellaneous Cost
28
LIST OF FIGURES Figure No.
Title
Page No.
1.
Prototype of scissor lift
10
2.
Worm & Worm Wheel
13
3.
Rack & Pinion
14
4.
Scissor Linkages
15
5.
3D model
6.
Link Design
23
7.
F.B.D
24
16-22
Abstract Scissor lifts (Aerial work platforms in general) are generally used for temporary, flexible access purposes such as maintenance and construction work or by firefighters for emergency access, which distinguishes them from permanent access equipment such as elevators. They are designed to lift limited weights. The contraction of the scissor action can be hydraulic, pneumatic or mechanical (via a lead screw or rack and pinion system). 2
The main objective of scissor lift is to save time required in building platforms or temporary lifts. Once the lift is constructed according to requirement, much time is saved .Its portability can enable the workers to use it at various working areas whenever needed . Considering the cost of lift it is suitable and more productive than making temporary platforms at construction sites with help of bamboo sticks or other materials. Maintenance cost is almost negligible as only lubrication is required for components.
1. Introduction
A scissor lift is a type of platform that can usually only move vertically. The mechanism to achieve this is the use of linked, folding supports in a criss-cross”X” pattern, known as a pantograph (or scissor mechanism). The upward motion is achieved by the application of pressure to the outside of the lowest set of supports, elongating the crossing pattern, and propelling the work platform vertically .Scissor lifts (Aerial work platforms in general) can generally used for temporary, flexible access purposes such as maintenance and construction work, which distinguishes them from permanent access equipment such as elevators.
3
1.1Necessity / Application of scissor lift
It can be used at construction sites for temporary or portable platform. o Eg. 1. Platform used for painting a wall at a certain height. 2. To elevate workers and their tools to a desired height for a job.
In emergency cases while extinguishing fire or by firefighter
As a vertical lift for supplying material or things on a certain height or upper storey.
4
1.2 Literature Survey.
There are three main types of aerial work platforms: boom lifts, scissor lifts, and mechanical lifts. They can be operated with hydraulics, pneumatics, or mechanically via screws or a rack-and-pinion system. They are either unpowered units, requiring an external force to move them, self-propelled with controls at the platform, or mounted to a vehicle for movement.
The aerial work platform invention is widely credited to John L. Grove, who was an American inventor and industrialist. However, even before JLG's first model, a company called Selma Man lift introduced a model in 1966.
As for John L. Grove, after selling his previous business, Grove Manufacturing, in 1967 he and his wife headed out on a road trip. During a stop at the Hoover Dam, Grove witnessed two workers electrocuted while working on scaffolding. Through this “tragic event” John Grove saw a large untapped market for a product that could put workers in the air more safely to perform construction and maintenance tasks.
When Grove returned home from his trip, he formed a partnership with two friends, bought a small metal fabrication business, and began designing concepts for the aerial work platform. The company was named JLG Industries Inc., and with the aid of 20 employees it released its first aerial work platform in 1970.
Aerial work platforms eventually began being designed with a variety of additional features. Many are now equipped with electrical outlets, compressed air connectors, and various other adaptations for tools.
2. Proposed Work 5
2.1 Proposed Prototype The machine consists of a worm and worm wheel, rack and pinion, shaft in a slot and the lift scissors (X linkages) . Rotation of handle attached to worm/worm wheel drives the system. The worm wheel and pinion are attached to a common shaft and thus, the pinion gets driven by the rotating worm wheel. The rack moves forward and this drives the main scissor mechanism at its bottom linkage. Thus the rotary motion to worm is converted into reciprocating / linear motion by using a pinion which moves the rack.
2.2 List of components and its function
Name
Function
Scissor lift platform Scissor linkages Rack and pinion arrangement
Platform to be elevated To provide elevation to platform To transfer power from worm wheel to scissor linkages
Worm and worm wheel
To transfer power from hand lever to rack with self-locking mechanism
Hand lever
To provide Mechanical power
6
2.3 Working Principle of prototype
7
As shown in image the handle is provided to provide clockwise rotational motion to the worm gear. The rotational motion of the handle is transmitted to the rotational motion in the same axis of worm as they are attached along the same axis. The rotational motion of the worm is transmitted to the worm wheel as rotational motion, but in an axis which is perpendicular to the axis of a worm. The worm rotates the shaft attached with it, which in turn rotates the pinion in the same sense as the worm. The pinion rotates in a clockwise direction which moves the rack in forward linear motion. The forward linear motion of rack results in forcing the scissor lift in upward direction. This upward motion is desired to raise the height of a load for any application.
8
The opposite happens when the handle is rotated in anti-clockwise direction. The anticlockwise rotation of handle leads to the downward motion of scissor mechanism. This downward motion is required when a load is to be lowered. As the load increases the effort required to raise the load is also increasing.
2.4 Technical parameter (Prototype) Lifting height
1.8 ft
Minimum height
100 mm
Length
1000mm
Breadth
300mm 9
Net weight
20.25Kgs
2.4Main Components and their design 2.5.1 Worm and worm gear
10
a) Worm & Worm Wheel A worm drive is a gear arrangement in which a worm (which is a gear in the form of a screw) meshes with a worm gear (which is similar in appearance to a spur gear). The two elements are also called the worm screw and worm wheel. The terminology is often confused by imprecise use of the term worm gear to refer to the worm, the worm gear, or the worm drive as a unit. Like other gear arrangements, a worm drive can reduce rotational speed or transmit higher torque. The image shows a section of a gear box with a worm gear driven by a worm. A worm is an example of a screw, one of the six simple machines.
2.5.2Rack and pinion
11
a) Rack & Pinion
A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move relative to the pinion, thereby translating the rotational motion of the pinion into linear motion.
2.5.3 Cross / Scissor linkages
12
a) Scissor Linkages A scissors mechanism uses linked, folding supports in a criss-cross 'X' pattern. Extension is achieved by applying pressure to the outside of a set of supports located at one end of the mechanism, elongating the crossing pattern. This can be achieved through hydraulic, pneumatic, mechanical or simply muscular means.
3.6 Drafting and 3D model
13
14
15
16
17
18
19
2.7 Calculations Link Design: Assumptions Max. Height (AC) = 600 mm
10o< θ < 110o
Max. Angle (θ) = 110o
Min. Angle (θ) = 10.0o
We’ve to find Length of Link (AB) =? If we design the scissor for single stage:-
In OO’A θ O' A sin = 2 OA O' A= OA=
AC 600 = =300 mm 2 2
AB 2
AB=732.5 mm
Total length of material required (l' )=2× AB=1465 mm For Double Stage:-
20
O' A=
AC =150 mm 4
∴ AB=356.4 mm l )=4 × AB=1425.6 mm Total length of material required ¿ < 1465 mm In case of double stage we can save material so we’ve chosen to go for double stage scissor mechanism.
FBD of Link: 21
Solution:
∑ F y =0,
−W + R y 1 + F y =0 2
∑ F x=0,−R x1 +F x=0 l
l
W
∑ M B =0 , F y × 2 cos θ+ F x × 2 sin θ− 2 l
l
×l cos θ=0
W
∑ M E=0 , R y 1 × 2 cos θ−R x 1 × 2 sinθ + 2
×l cos θ=0
When W = 50 N
Θ Fx Fy Rx1 Ry1
By assuming θ we get these values: 10o 20o 30o 1131 550 346 200 200 200 1131 550 346 0 0 0
40o 238 200 238 0
50o 168 200 168 0
60o 115 200 115 0
22
*Values approximated to near values. All the forces are expressed in Newtons (N).
2.8 Estimate Time required Sr.No Task 1. Design study
2.9
No.of days 15
2.
Mechanism study
15
3.
Design
30
4.
Manufacturing
20
5.
Testing
5
6.
Eliminations of errors and faults
10
Cost Approximately 11000-12000 Rs
23
Cost Estimation Cost of Material: Sr no 1
Name of Material
Qty
Material
Cost of material
manufacturing
Scissor lift Arms
6
MS
3000
2
Lifting platform
1
MS
1200
3
Base Plate
1
MS
1200
4
Bars
6
MS
900
Laser cut & Drilling Laser cut & Bending Laser cut & Bending Machining
5
Handle
1
MS
150
6
Shaft
1
MS
230
Total
6680
Machining
Cost of Standard parts: Sr no 1 2 3 4
Part Rack and pinion Worm and wormwheel Rack support Pinion support blocks
Quantity
Cost
1 1 1 2 Total
1500 1150 150 450 3250
Miscellaneous Costs: Sr no 1 2 3
Details Fasteners Accessories Painting
Cost 300 1100 150 24
4
Transport Total
250 1800
3. Risk management While operating scissor lift there are chances of sudden collapse of platform due to overload. To eliminate this risk a locking system should be provided which can stop motion of linkages when in locked position In prototype as worm and worm wheel is used. It doesn’t rotates reversely unless it is operated by handle.
25
4. Advantages and Disadvantages Advantages: •
The scissor lift has a unique mechanism which uses worm and worm wheel .This mechanism provides a self-locking system which makes the scissor lift completely safe for use.
•
Unlike the hydraulic systems, this mechanism has to be driven to bring the platform back down. This gives us the opportunity to use this lift as a machine part for accurate elevation.
Disadvantages:
Scissor lift occupies substantial floor space which makes it unsuitable for smaller applications.
Height of the elevation is limited. Effort required to lift material increases with increase in weight. Periodically lubrication is to be done for smooth working.
26
5.Feasibility study 5.1 Economic feasibility Considering the cost of lift it is suitable and more productive than making temporary platforms at construction sites with help of bamboo sticks or other materials. Maintenance cost is almost negligible as only lubrication is required for components.
5.2 Technical / operational feasibility There is no need of external support for the platform as the linkages itself work as supporters. Only single worker is required to operate the lift and thus it saves man power as compared to temporary platforms which need labors while being construct.
6. Conclusion As a whole, it has been concluded that this System if very beneficial according to economic as well as technical point of view. Once the lift is made there is no wastage of time as compared to time required for temporary platforms in assembly and disassembly while shifting them at other work stations. Hence the system is feasible. 27
7.Future Scope •
We can increase contact force between shaft and pinion so as to prevent slipping and allow lifting of larger weights.
•
Number of plates in the scissors can be increased to improve the height to number of rotations ratio.
• •
Hydraulic actuators can be used for heavy applications.
28
29
\
30
31
32
