Mechanical
Creating a statically determinate system
Fixing the gearbox output shaft to the cable drum creates a rigid,
statically indeterminate mounting (Figure 1) — making perfect
alignment and leveling critical during the assembly process.
Both, however, are difficult to achieve in practice. Additionally,
mounting inaccuracies, structural deformation and wear in
moving parts create additional forces in the gearbox output
shaft. These forces occur in alternating bending loads and can
lead to shaft breakage and severe damage to bearings and gears.
This is where barrel couplings come into play.
Barrel couplings perform the function of an articulated
joint, making the connection between the cable drum and
gearbox statically determinate (Figure 2) and avoiding high
bending moments. They also prevent angular and axial shaft
misalignment caused by mounting inaccuracies, structural
deformation, and normal wear and tear. Without the flexibility
provided by these couplings, the forces generated by the
alternating bending loads can lead to shaft breakage and other
costly, disruptive damages.
Figure 3 illustrates the mounting of a barrel coupling in a
typical lifting mechanism. Because this coupling allows axial
displacement, a self-adjusting bearing must be mounted—fixed
laterally—at the opposite end of the drum shaft. In special
applications, however, the barrel coupling can be designed as an
articulated joint that withstands axial forces by itself.
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120 August 2019 http://www.designworldonline.com DESIGN WORLD
The design advantages of barrel couplings
The barrel coupling’s design offers several benefits:
- Due to the profile of the barrels and teeth, the coupling is^
subjected to lower bending stresses at the bottom of the
teeth — improving protection against bending and
radial loads.
- The larger contact surface between the barrels and teeth^
also allows radial loads to be better distributed, which
increases the lifespan of the coupling.
- The use of high-strength materials, such as steel, increases
the coupling’s transmission capability without requiring
a change in the design or dimensions. As a result, a smaller
coupling size can often be selected — leading to
improvements in energy efficiency due to the lower weight
and acceleration forces.
- Internal and external covers, each equipped with a lip^
seal, prevent foreign matter from entering and lubricant
from escaping.
- A wear indicator attached to the external cover lets you^
check the wear and axial position of the housing relative
to the hub. Also, markings facilitate easy reassembly
should you need to dismount the coupling.
Claw and disc couplings
In addition to gear couplings, claw and disc couplings can be used to
connect the drive motor and gearbox in crane and hoist applications.
Like the gear coupling, these other two options are designed to
compensate for angular, radial, and axial shaft misalignment:
Claw coupling — Torsionally flexible, these couplings transmit
torque via elastic buffers made from Perbunan (Pb), which are
joined together as an intermediate ring. This ring dampens shocks
and torsional vibrations, is resistant to oil, and can handle a wide
temperature range. Because of their dampening capabilities, claw
couplings are preferable in many crane applications.
Figure 3. Mounting of the barrel coupling in a lifting mechanism.
Figure 1. Rigid gearbox — drum connection
(statically indeterminate).
Figure 2. Gearbox — drum connection using
barrel couplings (statically determinate).
Mechanical (Ringfeder) 8-19_Vs4_MS.LL.indd 120 8/5/19 2:18 PM
