Scissor Jack Design Project GE 410 Fall 2005 Jim Ramirez David Hettinger

Instructor: Hall 12/05/2005 ii

ABSTRACT

Scissor jacks are simple mechanisms used to drive large loads short distances. Thepower screw design of a common scissor jack reduces the amount of force required by the userto drive the mechanism. Most scissor jacks are similar in design, consisting of four mainmembers driven by a power screw. In this report, a unique design of a scissor jack is proposedwhich is very easy to manufacture. Each member, including the power screw sleeves, is made ofthe common c-shape. This eliminates the need for machined power screw sleeves, whichconnect the four members and the power screw together. The manufacturability of the proposedscissor jack lowers the cost of production.

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TABLE OF CONTENTS

Title Page…………………………………………………………………………………... i Abstract…………………………………………………………………………………….. ii Table of Contents…………………………………………………………………………... iii

Introduction………………………………………………………………………………… 4 Proposed Design…………………………………………………………………………… 5 Figure 1: Labeled Scissor Jack Design…………………………………………………….. 5 Table 1: Design Criteria……………………………………………………………………. 9 Conclusions and Recommendations……………………………………………………….. 10 Appendix A: Drawings…………………………………………………………………….. 11 Appendix B: Calculations and Assumptions for Components 2, 4, 6, and 8……………… 12 Appendix C: Calculations and Assumptions for Components 3 and 7……………………. 13 Appendix D: Calculations and Assumptions for Components 1 and 5……………………. 14 Appendix E: Calculations and Assumptions for Component 9……………………………. 15 Appendix F: Calculations and Assumptions for All Pins………………………………….. 16 Appendix G: Calculations and Assumptions for Crank Handle…………………………… 17 Appendix H: ANSYS Force Analysis……………………………………………………... 18

iv

Introduction

The most basic scissor jack design is truly engineering at its finest. With the power tomagnify input forces, scissor jacks allow us to raise vast loads using only a fraction of the forceordinarily needed. Our goal in this project is to design an efficient scissor jack capable of raisinga 2000lb load. As a screw-driven mechanical system, the jack will be manually operated andhave at least 7 inches under load. The design will be transportable and storable, have aremovable crank handle, and operate with a factor of safety of n = 2 using standard mechanicaldesign methods for all components. The design itself has gone through multiple stages ofdevelopment. We have taken several possible failure modes into account and are confident thatour design is efficient and safe.

Proposed Design

Summary

The scissor jack design, shown in Figure 1, consists of four main lifting members, fourconnection members, a power screw and a crank. Members 1 through 8 are all primarily c-shapes with ideal pin connections. Members 1 and 5 both have additional details to account forthe contact surfaces. The power screw is single threaded with a collar at the member 3connection. All members are 50 ksi strength steel with the exception of the rubber grip on thecrank. The following is a summary of the design features for our proposed scissor jack. Detailsof the design specifications and failure criteria can be found in the attached appendices.

Main Lifting Members:

These members are made from simple c-shapes. The web of the members is cut out nearthe pin connections to allow proper serviceability of the scissor jack at its maximum andminimum heights. Members 4 and 6 have ideal gear connections to balance the load between theleft and right side.

The flanges of the channels are to wrap around the flanges of the sleeve members. Thelifting members are greater in length and are subjected to compression. Lifting member flangeson the outside of the sleeve flanges is to compensate for slenderness ratio by increasing themoment of inertia of the lifting members.

Sleeve Members:

The sleeve channels are to open inwards as shown in Figure 2. This is so the flanges aresubjected to tension instead of compression. The bending moment from the power screw createstension on the inner edge of the sleeve and compression on the outside edge. Tension along

flanges on the inside prevents the possibility of localized bucking in the flanges fromcompressive forces.

Figure 2: Orientation of Sleeve Channels to Prevent Localized Buckling.

Additionally, the threaded sleeve section is to have additional thread surface area, shownin Figure 3. These additional threads safely transmit the stress from the power screw to thesleeve. Threading the thickness of the web of the channel would not be sufficient for reasonablepower screw diameters. This addition is only made on the threaded sleeve section and not on thecollared sleeve section. The collar transmits the stress safely to the c-shape.

Figure 3: Addition to C-Shape to Provide Adequate Threaded Area.

Contact Members:

The members that make contact with ground and the service load are members 1 and 5respectively. Member 1 has additional flanges to provide a stable base for the mechanism whileservicing the load. Member 5 has an attached plate atop to provide sufficient contact area. Mostscissor jacks have ridges which lower the area of contact. This causes stress concentrationswhich can damage the underside of a car.

The Power Screw:

The Power Screw is single threaded with a collar on the side in contact with Member 3.The collar is assumed to be frictionless and the power screw has been designed to be self-locking. The primary raising method is through the power screw’s hook coupling which iscommon to most scissor jacks. Incorporated into our proposed design is an option for asecondary raising method. The collar on the power screw doubles as a bolt with a hexagonalhead. In a situation where the main hook coupling becomes inoperable, a standard socketwrench can be used on the hexagonal nut to raise the mechanism.

Design Criteria

The design checks used in the design of the scissor jack are summarized in Table 1. The

criteria are organized by failure mode with the applicable members identified.

Table 1: Design Checks for Different Failure Modes of Members

Conclusion and Recommendations

Our proposed design is similar to common scissor designs in some aspects, but alsoadvantageous in others. Similar to others, our proposed design can safely raise a load of 2000lbs to the required heights with relative ease on the user. Unique to our design, however, is themanufacturability of our design, which is much simpler. Since only c-shapes are utilized, bulkmaterial can be more efficiently purchased and used. Also, less machining is required sincethere are no complex sleeves for the power screw. Only simple attachments which can bewelded on are proposed. Therefore, when compared to similar scissor jack designs that performequally as well, our proposed design is recommended for its manufacturability and lower cost.

Appendix A: Drawings

Note: All Drawings are in Inches

**See ANSYS printouts in Appendix H: All force analysis support can be found there

*See “Appendix C Support” for supporting calculations (not computer-generated)

Appendix H: ANSYS Force Analysis

Note: Power Screw is composed of members 9 and 10 in ANSYS repor