34
FEaturE namE
comminution
Properly tightened
Mill liner manufacturer ME Elecmetal and bolt provider Valley
Forge & Bolt Manufacturing Co have joined forces to provide a
best practice guide for bolt tightening in mill-lining applications
Mill-liner bolt
and washers
“Up to
34% of
unplanned
downtime
in an SAG
mill is
caused by
loose or
broken liner
bolts”
S
ome studies estimate that up to
34% of unplanned downtime in a
semi-autogenous grinding (SAG)
mill is caused by loose or broken liner
bolts. This downtime can mean up to
US$380,000 per hour in non-sales costs
to a mining company.
Loose or broken bolts can also lead to
bigger issues such as grinding-mill
damage and personnel injuries, so it is
important to study the method used to
tighten liner bolts in order to avoid
undesired consequences.
Properly tightened bolts provide the
clamping force or load needed to keep
mill liners from moving. In order for the
bolts to do their job, the load applied to
them must be greater than the opposing
forces (gravity and the weight of the
charge), which are trying to pull the
liners off of the mill shell.
For many years, torque applied to the
bolts has been the primary focus of liner
installation. Results have varied widely,
to bolts during liner installation is by
using load-verification equipment. A
liner bolt, sealing washer and hex nut, as
shown in Figure 1, should be tested as
an assembly in calibrated torque-to-tension ratio equipment (shown in Figures 2
and 3).
Torque values required to achieve the
proper clamp load can vary significantly
based on many factors, which is why the
calibration of installation tools and
verification of the load being applied to
the bolts are critical.
rEtiGHtEninG ScHEDuLE
with the use of bottom or top lock nuts,
worn tools, mismatched nut and bolt
threads, reuse of bolts or nuts, use of
lubrication, lack of consistent plant air
pressure, and varying amounts of rubber
in the bolted joints. In many applications,
relying on just torque has resulted in
loose liners and bolt problems.
The preferred method to ensure that
the proper clamp load is being applied
Danafloat™ flotation reagents
Bolts are ready for retightening with the
first few operational mill revolutions. The
reason that ME Elecmetal recommends
bolt retightening is due to the
embedment factor that affects all
fastening systems. Embedment occurs
with any slight mismatch between the
bolt and nut threads, realignment or
‘settling’ of the bolt in the bolt seat, and
the absorption of torque by other factors
such as lubrication and forging-related
features on some bolt heads. The result
is the loss of clamp force, even after a
few mill revolutions.
Retightening should take place within
eight hours of liner installation. With a
change in mill direction on bi-directional
mills, retightening should take place
within eight hours or sooner. Prolonged
retightening or uncalibrated torquing
can be detrimental to liner bolts and
cause bolt failure and/or liner failure.
BoLtinG ProGrammE
Mill liners are not designed to be moving
parts; long-term exposure to loose
fasteners can be detrimental to both
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Fig 1 (above): the
components of a bolt
fastening
Fig 2: (above right)
Clarkester torquetension tester
Fig 3 (right): Skidmore
Model K tensioner
July / August 2014
www.
.com
comminution
mills and liners, and can result in
unscheduled downtime. Inspections of
wrenches, air systems and complete
bolting programmes are designed to
support grinding operations.
Tightening tools and calibration
equipment should be available for
bolting problems as well as support for
routine maintenance planning needs.
It is not recommended to reuse bolts
or nuts. The cost of bolt or nut
refurbishment versus the potential
problems or uncertainty in mill
operations that reused nuts and bolts
can cause does not normally outweigh
the cost of a new fastener.
Friction can, and usually does,
increase between reused bolts and nuts,
resulting in an improper load being
applied to the bolt with the same given
torque – resulting in undesirable
performance of the fastening system.
• B olt retightening is highly recom-
35
Mill-liner bolt kit
mended due to the embedment
factor that affects all fastening
systems;
M
ining companies should set up
mill-liner bolting programmes for the
early detection of bolt-related
problems; and
T
he cost of bolt or nut refurbishment
does not outweigh the cost of a new
fastener when considering the
potential problems that reuse can
cause.
loose liners and bolt problems;
“The cost
of bolt or
nut refur•
bishment
does not
outweigh
installation is by using load verification
equipment;
the cost
Table 1: Recommended clamp loads for common bolt sizes of a new
Required clamp load for ASTM A449/ Required clamp load for A193 B7 equiv/
fastener
Bolt diameter
grade 5 equiv/class 8.8 (lb)
class 10.9 (lb)
when
1in
50,100
64,000
considering
1¼in
69,000
85,000
the
1½in
108,000
141,000
1¾in
110,000
165,000
potential
2in
140,000
183,000
problems
M24
50,000
64,000
that reuse
M30
69,000
85,000
can cause”
M36
108,000
141,000
the proper clamp load is being
applied to bolts during liner
M42
M48–M52
Summary
• R elying on just torque has resulted in
• T he preferred method to ensure that
•
110,000
140,000
165,000
183,000
Authors: Jeff Washburn, director of western North American sales at ME Elecmetal, and Bret Halley, chief operating officer at Valley Forge & Bolt
Manufacturing Co. See: www.me-elecmetal.com/en and www.vfbolts.com
MINING TECHNOLOGY WORLDWIDE
BALL / ROD / SAG / AG MILLS
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Whether for grinding or beneficiation, CEMTEC provides
the right solution for customers in the mining industry.
Whether for wet or dry applications, CEMTEC will meet
customers’ requirements regardless. Projects undertaken at
CEMTEC range from single machines to turnkey plants, from
engineering to commissioning and after-sales service.
Competent employees and professional facilities, such as the
workshop and the laboratory which enable appropriate product
investigations and test runs, ensure successful fulfillment of
the relevant assignment.
Interested in hearing more about CEMTEC? Please contact us!
CEMTEC Cement and Mining Technology GmbH, Ennshafenstraße 40, 4470 Enns, Austria ( +43/7223/83620-0, - info@cemtec.at
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