Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION OF THE INVENTION
CONSTANT FORCE LEVER BRAKE SHOE ASSEMBLY
FOR RAIL CLAMPS
Developed by: Dimitre Mintchev, P.Eng.
Phone: (604) 983 2835
Address: 201-125 19th Street West
North Vancouver, BC, V7M 1X4
E-mail: dimiter.mintchev@gmail.com
Rev. 0
04/12/2013
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TABLE OF CONTENTS
1. ABSTRACT 3
2. DESCRIPTION 4
2.1 Background of the Invention 4
2.2 Brief Summary of the Invention 4
2.3 Brief Description of the Drawings 5
2.4 Flaws of the Existing in Practice Rail Clamps 6
2.5 Description of the Preferred Embodiment 6
3. CLAIMS 9
4. DRAWINGS 10
LIST OF DRAWINGS
Fig.! Existing Brake Shoe - Clamp Engaged 10
Fig.2 Constant Force Lever Shoe Block Assembly 11
Fig.3 Constant Force Shoe Block Housing 12
Fig.4 Constant Force Cylindrical Shoe 12
Fig.5 Constant Force Brake Shoe 13
Fig.6 Constant Force Cylinder-Shoe Assembly 13
Fig.7 Constant Force Brake Shoe - Clamp Engaged 14
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2. DESCRIPTION
2.1 Background of the Invention
[0001] 2.1.1. Field of the Invention
[0002] The invention relates to rail clamps and, in particular, to spring
actuated, hydraulically
released brakes for cranes and other material handling equipment.
[0003] 2.1.2. Description of the Related Art
[0004] Rail clamps are well known and widely used. A typical rail clamp
comprises a pair of levers,
provided with a brake shoe block at the lever's first end to apply a clamping
force, and a toggle
mechanism or a cam follower at the lever's second end to apply an actuating
force. Until now, there
was a bias towards the actuating force only, see U.S. Patent No. W02009092156
Al, where a pair
of cam surfaces with a longitudinally decreasing slope are used to compensate
the decreasing spring
force, and to maintain a constant actuating force, as the cam is displaced
down from its nominal
position.. However, the clamping force is a derivative not only from the
actuating force, it is very
sensitive to the lever ratio, and in order to be maintained constant, a
special approach to the brake
shoe construction and engaging behavior is needed. Such approach is not
applied to the existing in
practice lever shoes, including the above cited patent. By using these
traditional shoes, the clamp
lever ratio can not be kept constant, when the rail is worn, and subsequently
a constant clamping
force can not be maintained.
2.2 Brief Summary of the Invention
[0005] The existing brake shoe blocks are of rectangular shape, and are
inserted into pre-machined
lever rectangular pockets in a way to keep the brake pad parallel to the rail
surface, when the clamp
is engaged. However, the parallel surface engagement is valid only in a case
of nominal rail size,
because the existing shoe construction does not allow angular variations. In a
case of worn rail, the
brake pad creates an angle to the rail surface, and the pad-rail contact
surface converts to a contact
line, located at the lower pad edge. This phenomenon decreases the clamp lever
ratio, decreasing the
clamping force substantially.
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[0006] It is an object of the present invention to provide a novel and
improved lever brake shoe
assembly, which compensates the spring force variations, as the actuating
spring extends and
contracts. The shoe assembly is constructed in a way to keep the engaged brake
pad surface parallel
to the rail surface, which ensures a constant lever ratio and a constant
clamping force, as the rail size
varies due to wearing.
[0007] There is provided a lever brake shoe assembly comprising: a lever; a
cylindrical shoe block
housing, machined at the first lever's end, including two pin holes; a
cylindrical shoe holder, hold
into the shoe housing by means of pins, allowing rotation; a serrated shoe
brake pad, mounted to the
shoe holder by means of holding slots. In case of variations of rail size, the
clamping force creates a
moment around the pad-rail contact line at the lower pad edge. This moment
rotates the cylindrical
shoe holder until the pad surface becomes parallel to the rail surface,
maintaining the clamp lever
ratio constant and thereby a constant clamping force.
2.3 Brief Description of the Drawings
[0008] In the drawings:
[0009] Fig.1 Existing Brake Shoe - Clamp Engaged, is a side elevation view
showing the
differences, when an existing brake shoe is engaged to a rail with a nominal
size, and to a worn rail;
[0010] Fig.2 Constant Force Lever Shoe Block Assembly, is a side elevation
view showing the
assembly's parts;
[0011] Fig.3 Constant Force Shoe Block Housing, is side and front elevation
views showing the
shoe block housing, machined into the lever;
[0012] Fig.4 Constant Force Cylindrical Shoe Holder, is side, front and top
elevation views of the
cylindrical shoe holder;
[0013] Fig.5 Constant Force Brake Shoe, is side, front and top elevation views
of the serrated brake
shoe;
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[0014] Fig.6 Constant Force Cylinder-Shoe Assembly, is side, front and top
elevation views of the
assembly comprising the cylindrical shoe holder and serrated brake shoe;
[0015] Fig.7 Constant Force Brake Shoe - Clamp Engaged, is a side elevation
view showing the
operation equivalence, when the constant force brake shoe is engaged to a rail
with a nominal size,
and to a worn rail;
2.4 Flaws of the Existing in Practice Rail Clamps
[0016] Fig.1 Existing Brake Shoe - Clamp Engaged, shows a typical existing in
practice rail clamp
lever assembly in an engaged position. The assembly consists of a lever 1, a
pivoting point (pin) 2,
an actuating point 3 at the lever's second end, and a shoe brake pad 5 at the
lever's first end. In order
to engage the clamp, an actuating force Fa is applied to the lever actuating
point 3. It causes the
lever 1 to pivot about the lever pivoting point 2 at an angle a, forcing the
shoe brake pad against the
rail 4, creating a clamping force Fc. Being preliminary machined at a certain
angle b, the shoe pad
surface becomes parallel to the rail surface, when engages, keeping the lever
ratio L2:L1 constant,
as it is shown on the left-hand side of Fig.!. However, the above scenario is
valid only when the rail
size is nominal. In a case of a decreased rail size, due to wearing, the pad-
rail parallel surfaces
scenario is not valid. The decreased rail width causes a bigger lever pivoting
angle a', which creates
an angle c between the engaging surfaces of pad 5 and rail 4. This angle
transforms the pad-rail
contact surface to a pad-rail contact line at the lower pad edge, which
transformation increases the
lever's arm length Li to Li', decreasing the lever ratio to L2:L1'. The
decreased lever ratio changes
the nominal clamping force Fc to a smaller clamping force Fc', as it is shown
on the right-hand side
of Fig. 1. Having for an example a typical for rail clamps nominal lever ratio
3:1, when the rail is
worn, the angled brake pad decreases the lever ratio to 2.5:1, and
subsequently:
- the clamping force becomes 17% less than the nominal one;
- the brake pad and rail contact surfaces are partially engaged.
2.5 Description of the Preferred Embodiment
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[0017] Maintaining a constant clamping force is a more complex task, than
keeping only the lever
actuating force constant. The other side of the problem is to keep the clamp
lever ratio constant,
when the clamp is engaged.
[0018] The goal of the presented invention is to combine into one device all
of the following
features in order to maintain a constant clamping force:
- to prevent the rail clamp lever ratio from decreasing, when the rail is
worn;
- to create a means to keep the lever ratio always constant, when the clamp is
engaged,
independently of rail size;
- to create a means, ensuring that the brake pad and rail contact surfaces are
entirely
engaged, when the rail is worn;
- to create a means, ensuring safe transfer of the clamping force to the rail;
- to take into consideration the construction of the existing in practice rail
clamps in order
easy application.
[0019] Fig.2 Constant Force Lever Shoe Block Assembly, shows the construction
of a lever and
shoe block, allowing the lever ratio to be kept constant, independently of the
corresponding rail size.
The assembly consists of a lever 1, a lever pivoting point (pin) 2, a lever
actuating point 3 at the
lever's second end, and a constant force brake shoe block at the lever's first
end.
[0020] The constant force brake shoe block comprises items as follows:
- a cylindrical shoe block housing 8, machined into the lever 1, shown on
Fig.2, Fig.3, Fig.7;
- a cylindrical shoe holder (a holding cylinder) 7, shown on Fig.2, Fig.4,
Fig.6, Fig.7, hold
into the shoe block housing 8 by means of holding pins 9;
- a serrated brake shoe 6, shown on Fig.2, Fig.5, Fig.6, Fig.7, mounted to the
holding
cylinder 7 by means of holding slots 11 and a holding pin 10.
[0021] Fig.7 Constant Force Brake Shoe - Clamp Engaged, shows the equivalence
in the clamp
operation, when the constant force brake shoe is engaged to a rail with a
nominal size, and when is
engaged to a worn rail.
[0022] In order to engage the clamp, an actuating force Fa is applied to the
lever actuating point 3. It
causes the lever 1 to pivot about the lever pivoting point 2 at an angle a,
forcing the shoe brake pad
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against the rail 4, and creating a clamping force Fc. The clamping force Fc is
transferred by the
cylindrical contact surface between the shoe block housing 8 and shoe holder
7, or can be
transferred by pin 9, if the magnitude of Fc allows. The brake pad and rail
contact surfaces are
entirely engaged.
[0023] In a case of a decreased rail size, due to wearing, the decreased rail
width causes a bigger
lever pivoting angle a', which initially creates an angle between the contact
surfaces of pad 6 and
rail 4. This angle transforms the pad-rail contact surface to a pad-rail
contact line at the lower pad
edge, as it is with the existing in practice clamps. The clamping force Fc,
transferred by the
cylindrical contact surface between the shoe block housing 8 and shoe holder
7, or by pin 9, creates
a moment around the pad-rail contact line. In the case of a constant force
shoe assembly, this
moment rotates the cylindrical shoe holder 7 until the pad contact surface of
the brake shoe 6
becomes parallel to the rail surface, maintaining the lever's arm length Li
constant, subsequently
the clamp lever ratio L2:L1 is constant, and thereby:
- the clamping force Fc remains constant;
- the brake pad and rail contact surfaces are entirely engaged.
