Arm unit for use in pantograph type jack

An arm unit for use in a pantograph type jack wherein a main driving cam having an effective cam face constituted with an arc of a circle is formed to one side plate at the mounting end of a U-cross sectioned arm on the side of a bearing member with the effective cam face being protruded forwardly and upwardly from the mounting hole for the shaft bearing member, whereas a driven cam with a relatively moderate and long cam face different from the shape of the main driving cam is formed to the other of the side plates such that, when a pair of opposed arms is an identical shape pivoted to the bearing member are rotated in the directions opposing to each at an equal angular velocity, the main driving cam and the driven cam for each of the arms are faced and brought into engagement under contact with each other.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns an arm unit of a jack and, particularly, it relates 
to an improvement of an arm unit for use in a pantograph type jack having 
four arms combined in a rhombic link structure for jacking the weight of 
automobiles. 
2. Description of the Prior Art 
As has been well-known, a pantograph type jack usually comprises a pair of 
lower right and left arms pivoted to a lower bearing member of a grounded 
base and a pair of upper right and left arms pivoted to an upper bearing 
member of a load receiver, in which the ends of the respective arms 
opposed to each other are rotatably pivoted to constitute a vertically 
deformable rhombic link. An actuation lever having threads formed 
therealong is rotatably screw-coupled to the rhombic link along the 
horizontal orthogonal line, so that the load receiver may be displaced 
vertically by rotating the actuation lever thereby deforming the rhombic 
link. 
The jack of the type described above is generally adapted such that the 
mating ends of the paired right and left arms are brought into rolling 
contact with each other under frictional resistance in the bearing 
portions so that the movements of the four arms are made equal to each 
other. For ensuring the rolling contact, the ends of the paired right and 
left arms attached to the bearing members are meshed with each other 
through a gear coupling in the prior art. However, since this structure 
requires steel material of a great thickness for forming gears, it can not 
meet the demand for reducing the weight of the jack. Further, although 
there has also been used a flange-reinforced thin plate material formed 
into the shape of gear teeth, the plate material undergoes severe 
fabrication upon forming the gear and, as a result, the material is 
degraded to reduce the strength, as well as high accuracy can not be 
obtained with ease in this case. 
In view of the above, rolling contact means that utilize paired cams 
brought into contact with each other have been employed in recent years. 
As such cam contact means, the present inventor has already developed an 
arm having a first cam of a heart-shaped curved face as an effective cam 
face which is formed at one of the side plate edges and a second cam of 
the same heart-shaped curved face as an effective can face which is formed 
in a different direction to the other of the side plate edges, 
respectively, of an arm shaft bearing member bended into a U-shaped cross 
section and has already filed an invention relating to such an arm as 
Japanese Patent Application No. 166343/1982. In the rolling contact means 
of this prior application, paired cam faces formed on both of the parallel 
side edges of one arm that is pivoted to one of shaft holes are brought 
into rolling contact with the identically shaped opposing paired cam faces 
of another arm that is pivoted to the other of the shaft holes. However, 
since each of the opposed cam faces is in a heart-shaped curved face with 
a small radius of curvature, no intimate engagement can be obtained 
between them thus failing to obtain satisfactory frictional resistance 
upon contact. Particularly, since the cam contact becomes insufficient in 
the angular range between the opposed arms, which is actually used most 
frequently, load balance between each of the arms is lost thereby causing 
dangers. 
OBJECT OF THE INVENTION 
The object of this invention is to provide an arm unit for use in a jack in 
which a main driving cam and a driven cam arm are disposed in parallel 
with each other to the rolling contact portion of each of opposed U-cross 
sectioned arms constituting a pantograph type jack, wherein the opposed 
top ends of each pair of right and left arms are well-engaged to each 
other under rolling contact over a wide range of rotational angle. 
SUMMARY OF THE INVENTION 
The foregoing object can be attained in accordance with this invention by 
an arm unit for use in a pantograph type jack having a rhombic link 
structure constituted by pivoting the upper and lower ends of respective 
right and left paired swingeable links to a pair of shaft holes that are 
formed while being spaced apart by a predetermined distance from each 
other to a pair of upper and lower shaft bearing members respectively in 
which each of the right and left paired swingeable links comprises upper 
and lower support arms pivoted respectively to each other at their opposed 
ends, wherein the end of each of the arms on the side formed with a 
mounting hole aligned with the shaft hole in the bearing member is shaped 
into a U-cross sectioned frame opened toward the inside of the rhombic 
link, a main driving cam that has an effective cam face constituted with 
an arc of a circle with the radius r and having the center at a point P 
situated above the axial line x in parallel with the bottom side of the 
arm passing through the center for the mounting hole is disposed to one 
side plate of the U-cross sectioned frame, with the effective cam face 
being protruded forwardly and upwardly from the mounting hole and, while 
on the other hand, a driven cam that has an effective cam face of a shape 
constituted with a curve that defines a region inside of a group of 
continuously moving traces of the arc-shaped cam face of the main driving 
cam plate from a region outside thereof when the pair of opposed arms and 
attached to a pair of the shaft holes and in the shaft bearing member are 
rotated at an equal angular velocity in the opposing direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
As shown in FIG. 1, a pantograph type jack according to this invention 
comprises a pair of lower right and left arms 4 and 4 which are rotatably 
pivoted respectively at their top ends to a pair of shaft holes 3 and 3 in 
a lower bearing member 2 integrated with a grounded base block 1 and a 
pair of upper right and left arms 4 and 4 each in the identical shape with 
that of the lower arms 3 and 3 and which are rotatably pivoted 
respectively at their top ends to a pair of shaft holes 3 and 3 in a upper 
bearing member 2 integrated with a load receiver 5. Corresponding base 
ends of the respective upper and lower right and left arms, 4, 4,--- are 
pivoted with each other so as to constitute a deformable rhombic link 
structure. 
An actuation lever 7 having a male screw 7' formed axially from the midway 
toward the top end thereof is rotatably journaled at one end thereof to 
the right pivotal end 6a of the rhombic link while screw-coupled at the 
male screw portion 7' thereof into a female screw formed to the inside of 
a shaft bearing 8 attached to the left pivotal end 6b of the link portion. 
In this way, the rhombic link is supported by the actuation lever 7 
disposed in the direction of the orthogonal line along the horizontal 
direction and expanded or folded vertically by the rotation of the 
actuation lever 7 thereby displacing the load receiver 5 vertically. 
Each pair of the upper and lower arms 4 and 4 is pivoted by means of 
mounting holes 9 to a pair of shaft holes 3 and 3 which are formed to the 
shaft bearing member 2 while spaced apart from each other by a 
predetermined distance D as shown in FIGS. 2 and 3. 
In the rhombic link of the embodiment according to this invention, the top 
ends of the opposing paired arms are brought into engagement under contact 
with each other by way of their cam faces. The term "engagement under 
contact" collective involves the cases where the top ends of the arms are 
caused to roll and slide while brought into contact with each other. 
This invention concerns an arm unit for use in the pantograph type jack as 
described above and, more specifically, it relates to a cam structure of 
the portions engaged under contact. As specifically shown in FIG. 2, both 
side walls of the arm 4 are bent into a U-cross sectioned frame. A main 
driving cam 10a having an arc of a circle as an effective cam face is 
formed to the edge on one side plate 4a, while being protruded forwardly 
and upwardly therefrom. While on the other hand, a driven cam 10b having a 
relatively moderate curve and different from the shape of the cam 10a is 
formed to the edge on the other side plate 4b so as to be brought into 
engagement under contact with the main driving cam 10a of the mating arm 
pivoted to the other shaft hole 3 of the bearing member 2 (refer to FIG. 
3). 
Specifically, as shown particularly in FIGS. 3 and 4, the main driving cam 
10a has, as an effective cam face, an arc forming a portion of a circle 
with a radius r having the center at a point P situated above the 
horizontal axial line x which is in parallel with the bottom side of the 
arm and passes through the center O for the mounting hole 9 of the arm 4 
and the cam face is formed so as to extend forwardly and upwardly from the 
mounting hole O. 
In a preferred embodiment, as shown in FIGS. 3 and 4, the radius r for the 
arc that constitutes the main driving cam 10a is set to about one-third of 
the distance D between the centers O.sub.1 and O.sub.2 for the pair of the 
shaft holes 3 and 3 of the bearing member 2, and the center P for the arc 
is situated near the vertical axial line y while being spaced upwardly 
from the center point O for the mounting hole 9 substantially by the same 
distance as the radius r. 
While on the other hand, the driven cam 10b has a relatively moderate 
curved face as shown, for example, in FIG. 4, which is brought into 
rolling contact with the main driving cam 10a of the other opposed arm 4 
pivoted to the bearing member 2. The outer circumferential profile of the 
driven cam 10b can be determined as a curve that defines a region inside 
of a group of continuously moving traces of the arc-shaped cam face of the 
main driving cam 10a from a region outside thereof when the pair of the 
opposed arms 4 and 4 pivoted to the pair of the shaft holes 3 and 3 in the 
bearing member 2 are rotated each at an equal angular velocity. The curved 
face of the driven cam 10b is actually determined, for example, as 
described below. 
As shown in FIG. 6, a sheet 11b used for determining the shape of the 
curved face on the outer circumference of the driven cam 10b is pivoted at 
the center O.sub.2, whereas a sheet 11a having the outer profile of the 
driving cam 10a is pivoted at the center O.sub.1. In this case, O.sub.1 
and O.sub.2 are defined as the centers for the shaft holes 3 and 3 
disposed in the bearing member 2 spaced apart from each other by the 
predetermined distance D as described above referring to FIG. 3. The sheet 
11a and the sheet 11b are rotated at an angular velocity equal to each 
other in the opposing direction (that is closing to or aparting from each 
other). Then, the arcuate cam face of the main driving cam 10a 
continuously draws a group of traces of moving arcs on the sheet 11b as 
the sheet 11a rotates relative to the plane of the sheet 11b. Thus, the 
effective cam face of the driven cam 10b that can be brought into 
engagement under contact with the cam face of the main driving cam 10a can 
be obtained as a curve 12 traced on the sheet 11b that divides the plane 
of the sheet 11b into a region defined inside of a group of continuously 
moving traces of arcs and a region defined outside thereof. 
The curved cam shape of the driven cam 10b brought into intimate engagement 
with the main driving cam 10a can thus be determined, and the main driving 
cam 10a and the driven cam 10b are formed in parallel integrally on both 
side plates 4a and 4b of the U-cross sectioned arm 4 on the side of the 
mounting hole 9. 
As shown in FIG. 6, the edge of each side plates 4a and 4b is turned back 
at least in the portion formed with the effective cam face so that a 
substantial cam width can be obtained as the turned back width W. On the 
other hand, the portion of the side plates 4a and 4b below the effective 
cam face is preferably cut away properly so that the portion may not 
hinder the rotation of the cams. The arm unit of the embodiment according 
to this invention can be prepared integrally from a steel sheet by 
punching and then applying bending fabrication. 
The operation and the effect of this invention will now be described below. 
The arm unit according to this invention is used in a state being assembled 
as shown in FIG. 1. Each pair of the opposed arms 4 and 4 pivoted 
respectively to a pair of shaft holes 3 and 3 in each of the upper and 
lower shaft bearing members 2 and 2 is rotated in such a direction that 
the opposed arms 4 and 4 are rotated in the directions opposite to each 
other while bringing the main driving cam 10a and the driven cam 10b into 
intimate engagement under contact to each other. 
In this way, since each of the upper and lower paired arms 4, 4,----- can 
be rotated with the main driving cam 10a and the driven cam 10b being in 
intimate contact with each other in each of the pivotal portions of the 
shaft bearing members 2 and 2, the four arms can surely be rotated each at 
an identical angular velocity and the load can be received in a 
well-balanced manner. 
The cam face of the main driving cam 10a is formed as an arc of a circle, 
while the shape of the driven cam 10b constitutes a moderate curved face 
extended forward of the arc in this invention. Accordingly, the main 
driving cam 10a and the driven cam 10b can be engaged more intimately to 
obtain more stable engagement under contact. Particularly, since the 
driven cam 10b has a moderate cam face extended forward of the main 
driving cam 10a in the arm unit of this invention as shown in FIG. 3, the 
length for the engaging contact is not identical between the driving cam 
10a and the driven cam 10b, but the length of the driven cam 10b is 
longer. Accordingly, when the two arms 4 and 4 are rotated at an identical 
angular velocity, the driven cam 10b, while sliding at a greater speed, 
intrudes toward the main driving cam 10a, thereby establishing a more 
effective engagement. Particularly, the embodiment shown in FIGS. 3 and 4 
is designed such that the driven cam slides most efficiently within range 
of the opening angle between the opposed arms from 160.degree. to 
60.degree. in the jack, which is most frequently used in the actual 
operation. 
Although this invention has been described referring to preferred 
embodiment illustrated in the drawings, various modifications and 
alterations are possible without departing the gist and the scope of the 
invention.