Keyless Bushes : Elimination of Keys / Keyways

[Michael Gomez]

Keyless Bushes : Elimination of Keys / Keyways ::

In any rotating equipment power is transmitted from the output shaft
of the generating source through a member like pulley/sprocket/gear/
coupling to the final location where it is transformed into work. For
generations, engineers have been used to seeing a key doing the most
important job of transferring power from the shafts to the various
components.

The idea of a key/keyway is so ingrained in the minds of users, that
many engineers / fitters refuse to accept the concept of power
transmission, without the necessity of these tiny obsolete products.

Mechanical power transmission is a very specialized field in the
totality of mechanical engineering and quite important, considering
that power generated through various costly processes has to be put to
useful work through effective transmission. components, without losing/
wasting efficiency. This aspect is very rarely considered in our
country, where I have witnessed on several occasions machines using A/
B/C/D section V-belts, without any consideration for the minimum
pulley diameter requirements - wasting quality and expensive V-belts
(of recurring expenditure), to reduce initial cost of CI pulleys!

Keys/Keyways ::

Keys/keyways have been an integral part of transmission, as the output
shaft of a generating machine is always cylindrical and the key acts
like a wedge between the shaft and transmission component.

Designing of keys has comparatively been an easy job, using shearing
forces for calculating the width/depth and shaft diameters increased
to accommodate the extra depth of the keyway. But machining has always
posed a problem. From shapers, milling machines to broaching machines
and EDMs, the need has been to manufacture a precise fit at an
economical cost.

The second most difficult part of using keys/keyways is assembling and
dismantling. At least assembly was not very difficult, invariably with
fitters and engineers around in the machine building areas. But for
maintenance or refitment, when the end user had to remove the pulley/
sprocket, curses were always heard around the plant!! In spite of all
these difficulties, engineers were always trying to design a new
product which could incorporate the key, but make it easier for
assembling/dismantling.

Taper Bushes ::

Taper Bushes Success was struck in the USA, where QD bushes and Taper-
Lock bushes (originally registered under the Dodge banner) were
introduced in the 1950s, making it easy to assemble and dismantle
components. The British used to call taper bushes, which needed keys
and tightening screws, as a "belt and braces" situation. This patented
product was later licensed to British and other manufacturers who
marketed the concept very successfully in other parts of the world.

These taper bushes needed a very precise taper (included angle of 8')
on the bush and the component, which were forced into each other by
using 2/3 tightening screws and a slit in the bush to accommodate the
'reduction' of diameters, squeezing the bush tightly, in between the
shaft and the component.


Although theoretically, a key was not needed, in all practical
applications, a keyway is always provided on these bushes. A very
clear tightening torque was needed for tightening the 2/3 screws, to
enable them meet the designed transmission torque (most of the Indian
manufacturers are not even aware of this and hence not popularised).

The 3rd, 4th and 5th half threaded holes were used as `jack-off'
holes, to separate the bush, from the component and the shaft, using
the just released 'tightening screws.

The QD bushes, working on the same principle, however, had a flange
outside the pulley/gear through which the bolts used to pass, making
them sturdier and heavier than the taper-lock bushes and ideal for
heavy duty/mining purposes. Unfortunately, both these taper bushes
needed keys/keyways to transmit the torque, but made it easier to
assemble and dismantle components from the shaft.

The disadvantages of these taper bushes, apart from the necessity of
keyways, was the run out (concentricity) levels which were based on
the taper surface machining to close tolerances.

The lateral movement which accompanied the tightening of the screws
was another negative aspect. Precise locationing was not an easy job
using these bushes.

Cone Clamping Units ::

Then in the 1960s, Ringfeder (of Germany) popularized the first and
real keyless bush enabling conveyor belt pulleys and heavy gears to be
mounted without the necessity of keys. Here, the most important
advantage was that the client could machine the straight bore in his
component without waiting for a branded product (pulley, sprocket,
coupling) with its machined taper surface and its inherent machining
defects. Germany and Japan caught on to using these cone clamping
units in most applications, considering the large advantages which
this product could offer.

These clamping units, inspite of the slight disadvantages of lower
concentricity levels, increased number of tightening screws. The
persistent lateral movement in addition to the slit totally and
finally eliminated the key/keyway.

The latest designs also offered models which could grip the 'shaft and
the hub together externally ideal for hollow shaft mounting. fixing
couplings/large flanges using a small huh diameter, making them
compact. The high transmission capacity of these units and the
comparative cost made them endearing to material handling equipment
manufacturers (where large shaft diameters were a rule, rather than an
exception).



Gradually realising the tremondous advantages, many other industries
like printing, packaging, machine tools started using them —
considering the accuracy which was, by that time needed in their
industries. With higher speeds, frequent reversals and the cost of
downtime being realised, industries started switching onto clone
clamping units, wherever necessary.



By the 1970s, several German and Italian manufacturers commenced
manufacturing of this product, thus adding to the availability and
competition, which started to bring down the prices and making them
more affordable. In spite of eliminating the key/keyway, the number of
tightening screws, the high tightening torque and once again the
necessity of `jack-off' holes were proving to be technically not
acceptable to industries which needed a precise fitting.

Into this scenario, in 1979, moved in ETP Transmission AB of Sweden
with a keyless bush, eliminating all the inherent disadvantages of the
then available cone clamping units. The scientist, Blaise Pascal
formulated the principle of pressure propagation in liquids, as: "A
liquid contained in a vessel which is subjected to pressure
distributes the pressure uniformly upon the walls of the vessel"


ETP has expanded this principle and applied it to the hub shaft
connections offering several advantages like compact design, fast
mounting, easy positioning, good runout, without damaging the contact
surfaces and finally easy dismantling. These qualities are becoming
important today and will be even more critical, in the future with
increasing pressure on reducing size of machines, higher precision,
better balance/runout, increased speeds and shorter downtime for
service. ETP bushes use a 'jelly' filled cylindrical bush to transmit
torque between the shaft and the component. The screws on the outer
flange are tightened gradually, thus increasing the pressure on the
"jelly" (between the inner and outer walls), which in turn gradually
forces the walls to expand - thus the outer wall clamping onto the
component and the inner wall holding onto the shaft - All, without a
key/ keyway.

Many of us forget or fail to notice the stresses which are being
created in a shaft, due to incorporation of the keyway. Figure 5
highlights this aspect. At the same time makes us realize that the
larger part of the shaft is not involved in any way, with the
transmission of torque.

Without the keyway, the shaft diameters can be reduced by 40% of
volume, leading to cost saving (not adding the cost of exactly
machining the keyway).

When tightening a ETP bush, the force exerted on the hub/shaft is so
uniform and smooth (as compared to the forces acting on mating
surfaces when a cone clamping unit is being tightened) that non-
ferrous products (like aluminium pulleys) can be used in conjunction
with an ETP bush (no other bush can be put to use with non-ferrous
products, unless very large hub diameters are designed to counter the
heavy forces involved).

As compared to cone clamping units and taper bushes, ETP does not have
a slit on the circumference which accumulates dust/bacteria and hence
ideal for food/pharmaceutical machinery applications In fact, once a
ETP bush is fitted onto a shaft/component, not even air passes through
the joint, allowing it to be used as a pressure sealing device.

Not satisfied with the design/development and successful launching of
the ETP Classic bush, ETP Transmission went on further to develop the
ETP Techno (just a single screw to tighten and release) for shaft
diameters up to 100 min. Being manufactured out of a single piece, the
price was considered too expensive for general engineering
applications and used only for critical applications, where
concentricity was of utmost importance.

Here again, ETP Transmission, came up with another solution - ETP
Express which also needs only one screw to tighten/release, but
manufactured out of two parts, thus reducing the cost, sacrificing a
little concentricity and making the product more affordable for the
general engineering industry. Again on an expansion spree, ETP
developed ETP Classic and ETP Express in stainless steel construction,
primarily to satisfy the needs of the pharmaceutical and food
industries, where liquids of different types and water are splashed
onto components which may be near to the packaging area.

At the Hannover Fair of 2001, ETP launched yet another product which
was found missing from its stable of products — ETP Octopus is used
for accurate locking of a sliding component after being positioned.
Compact design, without backlash and ideal for locking several units,
using existing hydraulic pump - mainly in the machine tool industry,
but also in steel rolling and other locations where movements have to
be locked and re-positioned, frequently. ETP Trans-mission AB of
Sweden aims to maintain their place as leaders in hydraulic fastening
and centering technology for many more years to come.

Conclusion ::

I have tried to highlight the advantages/disadvantages of various
products available in the Indian industrial market which have a
bearing on the total industry. Price is definitely a factor to be
considered, but, at least engineers and designers must be made aware
of the various options available to them, to eliminate keys and
keyways.


The author Mr.Michael X Gomez can be contacted via email at
trad...@vsnl.com or at No.5, South Phase, Ambattur Industrial
Estate, Chennai – 600 058. Tel : 44-26231267 / 26242184 / 65612184