18-10-2016, 09:37 AM
1459511056-CarJackscissorjack.pdf (Size: 3.41 MB / Downloads: 9)
Introduction
The standard scissor jack allows a person to be able to lift multiple tons over a certain height,
allowing it to provide assistance in changing car tires and performing other repairs on vehicles.
The height that the jack is able to lift the load depends mainly on the jackscrew mechanism used
in the design. With the given design requirements of a load of 4,500 lbs and maximum input
force of 500 N, the length and pitch of the jackscrew for the initial CAD model were determined.
In the preliminary calculations, the length of the jackscrew was designed to be 18 inches long
and the pitch of the jackscrew was designed to be 8 threads per inch. These calculations are
further elaborated on in the Preliminary Research section of this report. The other parts of the
scissor jack include: four identical arms, two identical hinges for the joint where the arms and
jackscrew meet, a foot to serve as the base of the jack, a top bracket to hold the arms together,
and a top bracket attachment to allow the jack to rest easily in one of the jack points under a car.
In order to meet the requirement of a minimum extended height of 12 inches, the arm lengths
were designed to be 7 inches long. The geometric reasoning behind this can be seen in the
sketches included later in this report, as well as the Preliminary Research section. All parts of the
jack were modeled individually in SolidWorks and then assembled using the appropriate mates.
Design!Vignettes
The top bracket of the jack was created with a simple design to serve the purpose of creating a
hinge with the upper arms of the jack. The basic shape of the bracket was a extruded rectangle
with the two longest top edges filleted to a radius of 0.5 inches. The outer edge of the extruded
rectangle was the offset inward to create a curve for the extruded cut that would hollow out the
rectangle, resulting in the bracket shape. The offset used for this curve was calculated in order to
maintain the bracket’s thickness of 1/8th inches. This thickness was chosen because it is the
minimum thickness of a plate of metal. The offset curve can later be edited if the thickness of the
bracket is deemed too thin. Next, all remaining edges of the bracket were filleted to give it a
polished look. After the fillets, the holes for the pins which would connect the arms to the
bracket were created using mirrored extruded cuts. Because these cuts were mirrored, changing
the size of one hole will change the size of all the others, keeping the holes consistent and
symmetric. Finally, the holes for the screws to attach the top bracket attachment were created,
using the same method, on the top of the bracket.
The center jackscrew was created in order to provide the axial direction in which the scissor jack
would be able to lift a load. The first step of the jackscrew was to make the threads. This was
done by first extruding a circle to the final jackscrew length. The end of this was then chamfered
to create ease for initial threading. A helix curve was then created with a pitch of 0.125 inches,
or 8 threads per inch, and to a length of the jackscrew minus the 0.20 inches that was chamfered.
This provided the path for which a small equilateral triangle would follow during a sweep
cut. Then, a hexagon was created and extruded to a height of half an inch for a circular loop that
would be used for the turn handle. The circular loop was created with a diameter of 1.5 inches to match the width of the hexagon and extruded around the mid-plane to 0.3 inches. The final step
was to make a cut extrusion of 1-inch diameter in order for the turn handle to fit easily.
The arms of the jacket were designed using the straight slot tool. The length was defined to be
seven inches to ensure that the jack could travel a minimum of twelve vertical inches. The width
of the arms was one inch. The slot was then extruded to a thickness of 0.125 inches. After that,
an extruded cut was performed to create the holes to pin the arms to the others parts of the jack.
The circles were concentric with the circular part of the slot and the diameter of all of the holes
was 0.125 inches. The base of the jack is the support of the assembly and was created from an
extruded rectangle. Two circles with 2.4 inches distance between their centers were sketched to
be the holes to pin the arms. To avoid stress concentration all edges were filleted. Then a boss
extruded was performed in the opposite direction to create the parallelepiped in contact with the
ground. This was extruded to 1.125 inches, which is half of the dimension length, generating half
of the base. The mirror tool was used and all of the features were mirrored. Changing dimensions
in a future design review are easier when the mirror tool is used.
Preliminary!Research
The design requirements for the scissor jack called for a minimum extended height of 12 inches.
In order to meet this goal, the arms of the jack were designed to be 7 inches long. When two
arms are hinged together, as seen in the assembly, they extend to roughly 14 inches depending on
the angle of extension. The geometric representation provided in the Sketches portion of this
report illustrates this calculation and reasoning.
The standard threads per inch of the jackscrew is 8 TPI. Knowing this and the requirements
above of a minimum load of 4,500 lbs and maximum input force of 500 N, the formula1 for
mechanical work can be used to solve for the distance at which the input force is applied:
!!"#$
!!"
= 2!"
!
Where Fload is the force the jack exerts on the load (4500lbs = 20,017 N), Fin is the rotational
force exerted on the handle of the jack (500 N), r is the length of the jack handle measured from
the screw axis to where the force is applied, and l is the lead of the screw (0.125in). This results
in an r value of:
! = 20,017
500 ∗
0.125
2! = 0.797 !"#ℎ!"
This would be an uncomfortable length for the operator to use and a distance of 6 inches was
used in the design. This results in an applied force of:
!!" = 20,017 ∗ 0.125
2! ∗ 6 = 66.4!
For most standard scissor jacks, the material used is described as “Heavy Duty Steel.” The
American Iron and Steel Institute (AISI) developed a classification system for different types of
iron and steel alloys. After some research, it was determined that a Nickel-ChromiumMolybdenum
steel alloy may be a possible material to construct the proposed scissor jack. This particular alloy has a classification of AISI 43202
. This steel alloy has been noted as an important
engineering steel in industrial use3
. However, if, after Finite Element Analysis, it is discovered
that the material affects the force calculations of the design, it may be changed to something
more appropriate.