Conveyor Idler Standard : SABS 1313
1.0 INTRODUCTION:
Conveyor idler standards, who sets them, what they are and what
they should be.
Prior to 1980 local conveyors were fitted with idlers manufactured
to various American and European standards. Each idler manufacturer
produced idlers with unique dimensions and roll fixing arrangements. In
general the company who was responsible for the original equipment
supply was ensured of a captive market for replacement spares. The
users, who over the years installed idlers emanating from various manufacturers
on different conveyors within their plant, had to keep a multitude
of non-interchangeable idlers and spare rolls in stock for
maintenance purposes. This resulted in the user incurring high inventory costs and suffering from
high obsolescence costs as critical dimensions changed with
time.
The need for a standard became obvious, and thus SABS 1313 was
created in 1980.
As to who sets the standard. SABS 1313 was intended to suit the
individual needs of all interested parties.
This was achieved by
utilising a forum whose delegates were drawn from:
- USERS
- SUPPLIERS
- NATIONAL STANDARDS ORGANISATION.
This paper discusses the current contents of SABS 1313, 1980 and
suggest areas where improvements are possible to further enhance
the standard.
2.0 SABS 1313 - THE CURRENT STATUS:
- 2.1. FUNDAMENTAL DEFINITION:
Prior to analysing the various elements it is important
that the nomenclature "IDLER" as defined in SABS 1313 is
clearly understood.
2.1.1 The idler is defined as the complete assembly comprising the base and or brackets and roll or
rolls.
2.1.2 The roll is defined as the revolving, cylindrical part of an idler, complete with shaft,
bearings and seals.
The basis of the definition is illustrated below:
In many cases this basic definition is not clearly understood or applied and often the purchaser's request
is for an idler although his requirement is for a roll only.
2.2 RANGE OF APPLICABILITY:
The existing South African Standard covers the dimensional
specification for both carrying side and return side conveyor
idlers for belt widths 400mm to 2400mm.
The following idler types are considered.
2.2.1. 3 Roll Trough and Impact in both offset and in-line
configuration.
OFFSET CONFIGURATION
IN LINE CONFIGURATION
3 ROLL TROUGH
OFFSET CONFIGURATION
IN LINE C0NFIGURATION
3 ROLL IMPACT
2.2.2. 5 Roll Trough and Impact in offset configuration.
5 ROLL TROUGH IDLER
5 ROLL IMPACT IDLER
2.2.3 Flat, single roll carrying idler.
FLAT CARRYING IDLER
2.2.4 Single Roll flat return idler.
FLAT RETURN IDLER
2.2.5 Two roll v Return idler.
V RETURN IDLER
2.2.6 "SPECIAL" return roll - more commonly known as underground or Colliery return roll. The face
length and shaft length of this type of roll are
longer than the standard for the specific belt size and was designed to
facilitate the training of the return belt in
underground applications. The nomenclature 'special' should not be
used in a standard and it is recommended that
this be replaced by 'extended'.
EXTENDED RETURN ROLL
2.3 DIMENSIONAL SPECIFICATIONS:
Having defined the various idler types consider now the
dimensions specified in SABS 1313 to ensure interchangeability.
2.3.1 IDLER:
The dimensions defining the idler are dependant
on belt size and roll type. The following dimensions are
specified:
2.3.1.1 CARRY SIDE:
-
-
|
MOUNTING CENTERS |
-
|
Dependant on belt size. |
-
|
FIXING DIMENSIONS
|
-
|
Dependant on belt size and roll series
The length and diameter of mounting
slot
is defined. |
-
|
TROUGH ANGLE
|
-
|
Note that the angles specified are
20, 25, 35 and 45 for
the offset
configuration and only 35 or 45 for the
in line configuration.
The problem with in line idlers is that
for smaller troughing angles the gap
cannot
be maintained. |
-
|
HEIGHT ABOVE BASE
|
-
|
Dependant on roll diameter and series.
Note that the roll height is different
for
in line and offset configuration. |
-
|
GAP BETWEEN ROLLS |
-
|
(IN-LINE) |
-
|
OVERLAP BETWEEN ROLLS (OFF SET) |
-
|
SPACING OF OFFSET ROLLS |
2.3.1.2 RETURN SIDE:
The same basic dimensions as per
carry side idlers are defined. The roll height below the base is now
defined as drop height. The dependency of roll diameter on roll height is removed by defining the drop height
as the dimension between the
support point and the center line of roll shaft. There are two angles specified
for the V return, 5 and
10.
2.3.1.3 BASE:
The configuration of the case is basically
defined by the configuration of the complete idler and only two other
dimensions are specifically defined.
2.3.1.4 THE ROLL:
-
The basic dimensions defining the roll are:
-
ROLL DIAMETER - Rolls in accordance with SABS 1313 are restricted to
those manufactured from tubing in accordance with SABS 657 Part III, which ensures minimum standards
of ovality and Straightness. The currently listed diameters are 102, 127, 152, 165 and 178mm. The
standard defines a minimum wall thickness with actual wall thickness being left open. it is
recommended that a range of standard wall thicknesess applicable to each diameter be created to eliminate the
current trend in variations to purchaser required wall thickness (e.g. 3,5; 3,8; 4; 4.5; 5; 6,3mm)
-
GAUGE LENGTH - distance between the inner shoulders of the flats of the
roll shaft ends.
-
SHAFT ENDS - defined by length and width of flats in either open end or
closed end configuration as illustrated above. These dimensions vary in accordance with the roll
series specified.
-
SERIES - is defined by the nominal shaft diameter. In general the series
should give an indication of the load carrying capacity of the roll. However the current definition is
not specific and it is generally accepted that "series" relates to the diameter of the shaft at its
ends and ensures correctness of fit between roll and bracket. For example, a roll having 30mm diameter
bearings but turned down at its ends to suit roll support for a 25mm diameter shaft would be termed a
series 25. This definition is required to be more specific.
-
The currently defined series are 20. 25. 30 and 40. There is a trend towards series 35 and the
range should be extended to include for this. Note that a series 30, 127 diameter roll does not form
part of the current Standard although many of these rolls are in use. The range should be further
extended to include for this.
2.4 PERFORMANCE
The Only topic related to performance in the current standard is
that of TOTAL INDICATED READING - T.I.R.
This is defined as; T.I.R. (max) = L/600 + 0,55.
With a maximum reading of 0,5mm at the roll ends.
As roll ends are not defined there could be misunderstanding as
to where to measure the T.I.R. e.g.
It is assumed that the reading at the roll ends is to be taken at
point 2. The point or range over which this measurement should be
taken must be defined by specifying dimension "e". The specification should also
include some allowance for the possibility of taking measurements
at points where surface irregularities of the tube exist.
Minimisation of roll runout is important in roll and indeed
conveyor belt performance in that:
-
A reduction in runout implies a reduction of out of balance forces acting on the bearings hence
resulting in improved roll performance.
-
Roll Runout has a significant influence on belt vibrations, which result in additional loads on
both the supporting structure and the idler rolls. Additional loads which are often neglected and
difficult to account for in the design process.
2.5 QUALITY:
The dimensions used to specify the idler, base or roll are
allowed to vary within specified tolerances.
The current specification defines the size of sample to be inspected
dependant on the lot size submitted for inspection.
3.0 EXTENSIONS TO THE RANGE:
Although the range discussed in section 2.2 is extensive, there
is a need, as established by purchaser requirements, for additional
items to be included.
Some of the more common, which could be relatively easily included
by utilising the existing roll dimensions and the addition of
relative belt line dimensions are:
- Two roll flat return idler:
- Two roll flat carry idler:
- Picking idlers:
- Trough Training idlers:
- Return Training idlers:
- Rubber disc return idlers.
3.1 COLLIERY REQUIREMENTS:
The largest single user of conveyor idlers is the underground
coal mining industry. Their need for continuous production and
frequent movement of conveyors between operating sections requires a constant availability of
replacement idlers.
The need for a specific standard applicable to idlers operating on
these conveyors has been partially addressed by the introduction of
the "special" return roll in SABS 1313.
Apart from the standard trough idlers defined in SABS 1313 there
are generally two other idler types utilised on the carrying side.
These are:
-
the suspended or garland type idler where the use of a base is eliminated and the rolls are
interlinked and supported on the structural elements by means of suitable fixings.
-
the underslung fixed form trough idler where the belt line is below the base mounting point.
TYPICAL GARLAND SYSTEM
TYPICAL UNDERSLUNG SYSTEM
Garland idler users have been plagued with the problem of
varying belt heights when using garland idlers supplied by
different manufacturers.
The illustrations show that interchangeability of idlers can be
ensured by defining:
- Type of fixing
- Mounting centers
- Drop Height
In fact the supporting structure could form part of the
specification by including:
- Dimension between trough and return belt
- Minimum return belt height
3.2 OTHER MATERIALS:
SABS 1313 currently covers only steel rolls produced ex tube to
SABS 657, the only variation being for impact idlers.
In the case of impact idlers variations in dimensions in rubber disc
diameters used by various manufacturers is included for by allowing
for a broad tolerance band in the specified belt height dimensions.
In order to ensure some conformity the diameter and possibly the
properties of the rubber compound used should be specified. There
has also been a trend to produce rollers ex polymer material and the specification
should be extended to include for non-metallic rolls.
4.0 IDLER STANDARDS - WHAT SHOULD THEY BE ?
A standard should be such as to ensure that the manufacturer and
the user have clear, unambiguous specifications which are based upon
realistic requirements which can be uniformly applied.
To summarise the standard should:
-
Ensure dimensional interchangeability.
-
Ensure that the product is suitable for the application i.e. performance and design standards.
-
Ensure that the product is manufactured to acceptable standards of quality.
As discussed in the previous section the question of dimensional
interchangeability is generally well covered in the existing SABS
1313. However, the greatest failing of the existing specification is that it can be considered
as ONLY a dimensional specification. There are basically no
guidelines as to design and performance standards nor are the
quality requirements adequately covered.
This has led to the creation of numerous individual user standards
which account for the missing specifications in the current SABS
1313. Where user standards are unavailable the idler supplier is generally asked to supply
idlers in accordance with SABS 1313 and the final decision as to
contract award is based solely on commercial criteria.
After all, "the items are produced to a nationally acceptable
standard and are therefore equal". A true statement, only if
performance, design and quality criteria were included in the SABS 1313 standard.
The following is a list of recommendations for inclusion or
amendment to the existing standard.
4.1 DESIGN STANDARDS:
4.1.1 IDLER LOAD:
The
importance of having a unified formula for establishing selection parameters has
been clearly
illustrated in
previous Beltcon papers (e.g. paper presented by A MATTHEE at BELTCON 5)
The formula
for idler load must account for the following elements:
- Mass of material transported.
- Mass of the belt.
- Mass of the roll.
- Additional loads due to vertical misalignment.
- Additional loads imposed in convex curve zones.
- Additional loads due to dynamic effect.
4.1.2 ROLL - SHAFT/BEARING SELECTION
CRITERIA:
Idler rolls
are generally selected on the basis of:
- minimum calculated bearing life.
- maximum allowable shaft deflection.
The two
criteria are interdependant in that the amount of deflection between the inner and outer race
of the
bearing has a significant effect on bearing life.
4.1.2.1 SHAFT DEFLECTION:
The shaft
deflection is dependant on the distance between roll support points and the distance between
bearing center line and the adjacent support point. Although the distance between support (G) is specified
in SABS 1313
(gauge length) dimension A is not constant and is dependant on the sealing system utilised
by the
various roll manufacturers.
The
following maximum allowable shaft deflections, based on bearing supplier's specifications and
allowing
for assembly tolerances, are recommended.
"SEIZE RESISTANT" BALL BEARING: 10 minutes
DEEP GROOVE
BALL BEARING C3 CLEARANCE: 6 minutes
TAPER ROLLER
BEARING: 2 minutes
4.1.2.2 SHAFT BENDING:
In critical
applications the selected shaft diameter (based on the deflection criterion) should be checked to
ensure
that the maximum allowable bending stress is not exceeded.
4.1.2.3 BEARING LIFE:
In general
the load carrying capacity of an idler, due to the dependency on belt speed, is defined by the
calculated
bearing life.
In general
the ISO formula for calculating bearing life, based on endurance limit, is used.
No limits
of acceptability are stipulated in SABS 1313 and manufacturers base their design on individual
user
specifications. The figures used in the South African market are in the range of 75 000 hours to
100000 hours.
These differ substantially from the European Specifications of 25 000 to 30 000 hours, with
50000 hours
being used in critical applications.
The European
figures are the more realistic as grease manufacturer's specified grease life does not
generally
exceed 30 000 hours. Calculated bearing life would obviously decrease with decreasing lubricant
efficiency.
There is a general reluctance amongst local users to accept the European standards. This is
probably due
to:
- lower
levels of conveyor installation maintenance.
- lack of
confidence in idler supplier meeting the required manufacturing tolerances to ensure optimum
bearing life.
It is
therefore suggested that the design criterion for bearing life be based on a compromise limit of
50000
hours.
In general
bearing failure in idler rolls occurs by the ingress of contaminants into the bearings and not by
fatigue
(endurance limit) failure.
This was the
basis for the design of the S.K.F "seize resistant" type bearing which shows increased life
characteristics
(when compared to the standard deep groove bull bearing range) when used in
contaminated
conditions. S.K.F have developed formulae, applicable to the seize resistant range, which
account for
this (wear) mode of failure. These formulae generally tend to yield more realistic results than
the conventional
bearing life formulae. As the use of these bearings, particularly in the series 25 idlers,
are in general
use, it is recommended that they form part of any design standard.
4.1.3 IDLER BASE:
The rigidity
of the idler base has a significant influence on idler and overall belt performance.
Most users have
recognized this and specify a maximum acceptable deflection of the load carrying member in their
specifications.
It is
recommended that general steelwork design practice be used and that the maximum deflection be
limited to the
lower of 1/360 or 5mm (where 1 = free length of load carrying member).
4.2 PERFORMANCE STANDARDS:
Once an idler has been designed to perform under the specified
load conditions and selected in accordance with the applicable
dimensional specifications the user's final selection should be based on some measure of
performance.
In general the relative performance of idler rolls may be
compared by testing for the attributes required for good
performance viz:
- Maximum ease of rotation
- Maximum seal efficiency
4.2.1 ROLLING RESISTANCE:
The rolling
resistance of an idler roll i.e. its frictional resistance to movement under load, is dependant on
the roll
diameter to bearing diameter ratio, the operating speed, the load on the roll, and factors
specifically
applicable to individual roll design such as internal resistance due to seal construction.
Maximum
values of rolling resistance should be specified so as to ensure the accuracy of conveyor belt
designs. It
is recommended that DIN 22112, which includes a table of maximum values based on roll
diameter be
used as a basis for creating a local standard. Modifications would be required to account for
the range of
locally available tubing which is different to that specified in the DIN standards.
4.2.2 SEAL EFFECTIVENESS:
Methods of
testing the effectiveness of the seal in dusty and wet conditions are also defined in DIN
22112. There
is however no definition as to the acceptable limits of contaminations when the roll is
subjected to
the specified conditions Thus the results obtained from the test may only be used as
comparative
figures between different types of sealing arrangements.
Ultimately
the selection of the sealing arrangement is a compromise between maximum seal efficiency
and
minimum rolling resistance and would be dependant on prevailing operating conditions.
Therefore
the only inclusion in the SABS specification should be a description of the methodology
required for
testing the seal effectiveness.
4.3 STANDARDS OF MANUFACTURE:
Having defined the required dimensions and applicable tolerances
and the minimum acceptable limits of design and performance, the
standard should also include specifications as to the acceptable standards of manufacture.
Items to be addressed should include:
-
IDLER BASE:
IDLER ROLL:
-
Tube material specification.
-
Shaft material specification.
-
Welding specification.
-
Bearing seat tolerance on the shaft.
-
Locating tolerance of bearing in bearing housing.
-
Maximum allowable shaft axial float.
-
Total Indicated Runout.
As previously discussed the reduction of Total Indicated Runout is important to the overall performance
of the idler and is probably the most debated issue between users and manufacturers. In establishing
mutually acceptable limits consideration should be given to:
5.0 CONCLUSION:
The local idler standard, SABS 1313 1980, has served its purpose
in establishing a basis for the dimensional specification, hence
ensuring the interchangeability, of conveyor idlers.
The standard is currently under review by a committee comprising
users, manufacturers and the South African Bureau of Standards. A
preliminary revised standard incorporating design, performance and manufacturing
specifications has been published and its eventual implementation
will ensure that the manufacturer produces a product which is
readily acceptable to the user.
6.0 ACKNOWLEDGMENTS:
The authors extend their thanks to the management and staff of
the Melco Group of Companies for their assistance in the preparation
of this paper.
