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 BindingConnecting links needed between center pivotsHow well will the 3

sides move together and keep top in line with bottom?

Consider an actual Car Jack Design

The screw is horizontaland moves up and down during lifting.

Consider a Conventional Scissor Lift

The shape is rectangular.Parallel scissors are rigidly connected at outerpivots. Legs are made ofrectangular 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: commercial@lerc.fr• 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