Car jack mast design update
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Car Jack Mast Design Update. Presented by Doug Eddy and Dr. Sundar Krishnamurty at UMass Amherst for Hoppe Tool on 7/16/10. Control of Scissors’ Motion. This mechanism is under constrained. Potential Remedy.

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Car Jack Mast Design Update

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Car Jack Mast Design Update

Presented by Doug Eddy and Dr. Sundar Krishnamurty at UMass Amherst for Hoppe Tool on 7/16/10


Control of Scissors’ Motion

This mechanism is under constrained.


Potential Remedy

  • Theoretically, the bottom pivots far apart shortens travel and close together affects stability.


Alignment without Binding

Connecting links needed

between center pivots

How well will the 3

sides move together and

keep top in line with

bottom?


Consider an actual Car Jack Design

The screw is horizontal

and moves up and

down during lifting.


Consider a Conventional Scissor Lift

The shape is rectangular.

Parallel scissors are

rigidly connected at outer

pivots. Legs are made of

rectangular tubing for

strength and stability.


Engineering Analysis

  • A simplified explanation is found at:

    • http://www.engineersedge.com/mechanics_machines/scissor-lift.htm

  • The information is verified and correct.

  • This reveals a design challenge with the multiscissor arrangement.

  • The screw force required is very large when the angle is small at the start of lifting.

    • The threshold power spec is 850 Watts.

    • Mechanical advantage with unequal scissor’s lengths is only 15-35%.


Engineering Calculation Results


Car Jack Potential Design Remedies

  • Longer scissors’ length

  • Increase retract height and scissor start angle

  • Longer pitch lead of screw and nut length

  • Must also minimize friction and consider those effects

  • Clevis mounted angle optimized drive pivot mount?


Strap Design

8m mast supporting 120kg (EW system)


Claims of the Belt Design

  • LERC S.A.

  • BP 10119 – 59732 SAINT AMAND LES EAUX CEDEX – France

  • Tel: +33 3.27.22.85.50 – Fax: +33 3.27.22.85.55 – e-mail: [email protected]

  • Internet: www.lerc-composites.com

  • Page 2/4

  • Main Advantages

  • • Resistance to bullet impacts: a bullet impact on a pneumatic mast

  • manufactured from light alloy or pultruted composite will make a hole that

  • will result in an air leak and in the mast collapse. It will also make a slight

  • crack in the matrix likely to break the tube. In a belt drive telescopic mast,

  • a bullet impact (see picture) will make a hole without affecting the mast

  • height. Moreover, the woven and crossed structure (Filament Winding -

  • FW) of the composite material prevents any crack in the tube.

  • • Height maintained at constant level when the mast is in erection for

  • an extended time : a pneumatic mast tends to go flat and therefore to

  • retract, which can result in a cutting off of the radio contact.

  • • Outstanding resistance to corrosion, chemical attacks and ageing ;

  • • Undeformability: the tube sections show no permanent deformation even after extensive use (strength

  • maintained, no ovalizing);

  • • Best ratio between deployed and retracted heights

  • • Lightweight and outstanding mechanical resistance

  • • No maintenance other than wiping or brushing to clean and for the telescopic masts, replacement of the

  • belt without dismounting tubes (can be performed on the field).

  • • Excellent resistance to environmental conditions (use of Epoxy resin and FW process): Sand, dirt,

  • dust, snow, ice will not cause degradation of mast performance. On mast with the new belt system, the belt

  • is fully inside the mast, protected against outside environment.

  • • No air tightness to ensure : no adjustments to make height maintained at constant level ;

  • • Manipulation with naked hands, even under cold or hot temperature ;

  • • Adaptability to the customer’s needs : the mast structure is computer designed (SAMCEF method)

  • • LERC proven experience: close to 60 years in the field of composite materials, 30 years in the

  • manufacturing of tactical masts and antennas.


Design Concerns with Strap Concept

  • Resistance of bending around small pins

  • Possibility of slipping at speed desired if not enough tension

    • Contact area on small pins

  • Power required increases with tension and the number of pulley contacts for simultaneous lifting.


Question for you?

  • How do you and the customers rank or weight the various features?:

    • Low power required

    • Small area, length

    • Low weight

    • High reliability and strength

    • Low sway and deflection, high stability

    • Low start height

    • High extended height

    • Low cost

    • High payload capability

    • High speed capability

    • High center clearance area and proximity for the cabling and basket below


Consider a Simple Straight Pulley Lift

Work output = mgh = 45 pound payload x 20 feet lifting height = 900 lb - ft

If the total system mechanical efficiency were 64% (although not realistic , for a system lifting from underneath)

Work input needed = 900 / 0.64 = 1406 lb – ft

= F x d = System pull force x System pull distance

This must be done within 15 seconds,

Power = Work done per unit time = 1406 / 15 = 93.75 ft – lbf / second

550 ft – lbf / sec = 1 Hp, so for a 93% efficiency motor 93.75 / (550 *0.93) = 0.183 Hp = 137 Watts


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