otraco-web-publications-batch-analysis-guidelines-february-2014.pdf

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TECHNICAL PAPER FITMENT BATCH ANALYSIS GUIDELINES

1 INTRODUCTION ....................................................................................... 1

2 BATCHING RULES ................................................................................... 12.1 Rule 1: Batch size/Fitment period ...................................................... 1

2.2

Rule 2: New tyre fitment axle ............................................................. 2

2.3

General ................................................................................................ 3

3 DOING THE ANALYSIS ........................................................................... 33.1 Doing the analysis before all tyres have been scrapped .................... 33.2 Checking that no batches exhibit axle fitment bias .......................... 43.3 Presenting the results ......................................................................... 5

Attachment 1 Biased/Unbiased Life Calculation Sheet

Attachment 2 Sample Batch Graphs


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1 INTRODUCTION

Fitment Batch analysis, referred to simply as Batch analysis in thisGuideline, is the most appropriate method of comparing the performance

of various tyre specifications used in a specific application on a minesite.By performance we mean tyre life (kilometres or hours) or tyre operatingcost (cost per kilometre or cost per hour).

Batch analysis is based on the grouping of tyres by application andfitment period, typically covering six or 12 months of new tyre fitments.The average life (or operating cost) of each specification of tyre used in theparticular application over the specified fitment period is calculated andcompared in order to rank the tyre specifications from best to worst.

For example we may have a fleet of Cat 793 haultrucks to which four tyrespecifications are fitted: Michelin 40.00R57 XDR2 C4, Michelin 40.00R57XDR2 B, Bridgestone 46/90R57 VRPS E1A and Goodyear 40.00R57 RM-4A+ 2H. Batch analysis will enable us to rank these four tyrespecifications, for this particular haultruck application.

Batch analysis is most commonly conducted on haultruck tyres but can bedone for any earthmover (EM) rubber-tyre machine type, e.g. loaders,dozers or graders.

The other grouping method for tyre life analysis is based on scrap (rather

than fitment) period. Scrap period analysis is appropriate for determiningthe overall tyre life trend for a particular application, but it can giveerroneous results if used to compare life by tyre specification particularly when a new tyre specification (or a new equipment fleet) isintroduced to the minesite. For details on the pitfalls of using Scrap periodanalysis to compare tyre life by tyre specification, refer to the Otracopaper entitled Proper Tyre Management Reduces Tyre Costs.

2 BATCHING RULES

There are two main rules that should be observed when conducting Batchanalysis. They are based on:

1. Batch size/Fitment period, and2. New tyre fitment axle

2.1 Rule 1: Batch size/Fitment period

Rule:Minimum batch size is 12 tyres (but preferably 16 or more)

The larger the size of a batch, the greater will be the statistical validity ofthe calculated average life of that batch. Otraco regards the absolute

minimum number of tyres in a batch as 12 tyres of the same specification.


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Batch size will generally determine the length of the batch fitment period.On a mine with a large fleet and high tyre usage we may be able to set thebatch fitment period as six months, e.g. covering all tyres fitted newduring the period say January 1stto June 30th. On a mine with a smaller

fleet and lower tyre usage we may have to set the batch fitment period as12 months, say from January 1stto December 31stto ensure that eachbatch being compared contains a minimum of 12 tyres1.

On a mine with a very large fleet and very high tyre usage it may bepossible to set the batch period as low as three months however Otracogenerally recommends setting the batch period as either six or 12 months.

It is usual to set six month fitment periods as January to June and July toDecember, but it may be appropriate in some cases (usually based onseasonal conditions such as temperature, snow or rainfall) to set

alternative periods, e.g. March to September and October to April. Where12 month fitment periods are deemed more appropriate (in order to ensurethe batches contain a sufficient number of tyres for statistical validity) theusual period would be from January to December or from July to June although again this could be varied if there was a compelling reason to doso.

2.2 Rule 2: New tyre fitment axle

Rule:Batches must be fitted in similar proportions by axle

In order to fairly compare different tyre specifications we must ensurethat the specifications have been fitted in an equitable manner.

For example if all (or most) of the tyres of one specification were fitted newto front axles of a haultruck fleet and all (or most) of the tyres of anotherspecification were fitted new to rear axles of the fleet, then we could notfairly compare the two tyre specifications as they would have effectivelybeen operating in two different applications.

Similarly, if all (or the vast majority) of the tyres of one specification were

fitted new to one axle (either all to the front or all to the rear), then wecould not fairly compare this batch with another batch in which asignificant proportion of tyres had been fitted new to both the front andrear axles.

For a fair comparison of tyre life (or operating cost) among the differenttyre specifications used on a haultruck fleet, either:

1Providing that each batch contains a minimum of 12 tyres it is acceptable to compare

the performance of batches containing substantially different tyre quantities, i.e. it wouldbe acceptable to compare one batch of 12 tyres with another of 26 tyres and a third of 33tyres.


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all the tyres of each specification should have been fitted new tofront axles and then run out on rear axles, or

the ratio of front to rear axle fitments for new tyres should havebeen similar for each tyre specification being compared.

If we have fitted all the tyres of a particular specification new to the rearaxle2then we can only fairly compare the life of these tyres with anotherspecification of which all the tyres had been fitted new, during the samefitment period, to the rear axle as well.

2.3 General

Because of these rules, we need to plan thepurchaseandfitmentof newtyres so that we are:

Fitting a minimum of 12 tyres for each specification in each fitmentperiod, and

Fitting each specification in an equitable manner by axle.

This requires planning at the time that tyre purchases are made and asnew tyres are fitted to the fleet.

If this planning has not been done prior to tyre purchase and fitment, thenit is likely that at the time it comes to conducting the Batch analysis,there will be problems. Some batches may comprise too few tyres for themto be statistically valid, or we may be comparing tyre specificationsunfairly because some have been fitted predominantly new to the front

axle while others were fitted predominantly new to the rear axle.

3 DOING THE ANALYSIS

If the above rules have been followed, then the task of conducting theBatch analysis will be made very much simpler. However there are stilltwo more things we need to be aware of when doing the analysis; they are:

Doing the analysis before all tyres have been scrapped, and Checking that no batches exhibit axle fitment bias.

3.1 Doing the analysis before all tyres have been scrappedIn an ideal world we would conduct Batch analysis for a specific fitmentperiod once all the tyres fitted new during that period have been scrapped.However that may lead to an unacceptable delay of our analysis (e.g. wemight need to make a tyre purchase recommendation now as we cannotafford to wait several more months until all the tyres have been scrapped).

2An example of a case where all tyres may be fitted new to the rear axle could be a batch

of cut resistant compound tyres that cannot be fitted new to the front axle for fear of lowlife heat separation, but which are expected to achieve exceptionally high life on the(lower TKPH) rear axle.


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The time from fitting the first tyre in a batch fitment period until the lasttyre of the fitment period has been scrapped could be two years or longer.And there are usually, for one reason or another, several tyres of one batchor another that remain in service long after the others batches for the

fitment period have been completely scrapped.

If we need to conduct a Batch analysis sooner rather than later, then it isusually acceptable to do so once each batch in the fitment period has atleast 80% of its tyres scrapped. The writer believes that the fairest way todo this is to examine the life statistics for each tyre that has not yet beenscrapped and assign an assumed scrap life to the tyre. This is not a perfectsolution but it is better than the remaining options of:

using the current life to date for these tyres (which is how Otracombatch analysis is calculated), or

excluding these tyres from the analysis.

Because we are making scrap life assumptions for a maximum of 20% ofthe tyres in the batch and because most of the tyres not yet scrapped arelikely to have achieved a relatively high life to date, it is not uncommon forthe calculated batch life (including assumed scrap lives for those few tyresnot yet scrapped) to be within 1% of the final scrap life of the batch. Hencethere is little or no downside to conducting the analysis earlier (while afew tyres remain in service) enabling us to recommend to the clientwhich are the optimum tyre specifications to be purchased in future rather than waiting several more months until all the batches being

compared have been completely scrapped.

3.2 Checking that no batches exhibit axle fitment bias

The final step that we need to take is to ensure that none of the batchesbeing compared exhibits an unacceptable level of axle fitment bias.

If all the tyres in all the batches that we are analysing and comparing hadbeen fitted new to the same axle3(either all to front axles or all to rearaxles) then it is unnecessary to conduct an axle fitment bias check.

However if some tyres in a batch had been fitted new to the front axle andsome fitted new to the rear axle, then we should conduct an axle biasfitment check to ensure that there is no significant level of bias that wouldpreclude a fair performance comparison with other batches.

The normal means of calculating batch average life is to simply computethe arithmetic average of the lives of tyres comprising the batch; Otraco

3Keep in mind that we cannot fairly compare the life of one batch of which all (or most) ofthe tyres had been fitted new to the one axle, with another batch of which all (or most) of

the tyres had been fitted new to the other axle (or in significant proportions to bothaxles). These two batches of tyres would have been effectively operating in differentapplications.


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calls this average: biased life (as it does not take into account any axlefitment bias). Unbiased life is the average life of the tyres in the batchafter adjustment has been made for any axle fitment bias. Themethodology and formula for calculating unbiased life is contained in the

aforementioned Otraco paper Proper Tyre Management Reduces TyreCosts. If the calculated values of biased and unbiased lives for a batchare within 2.5% of each other then we know that the tyres in the batchhad been fitted in an essentially unbiased manner in terms of axlefitment; the unbiased life of this batch can then be fairly compared withthe unbiased lives of other batches fitted in the same application duringthe same fitment period. If the biased and unbiased lives vary by morethan 2.5% then it is safest to exclude this batch from any performancecomparison.

Otraco has developed an Excel worksheet which calculates biased and

unbiased life and indicates whether the variation is within the tolerance ofa batch fitted without axle bias. An example is shown in Attachment 1.

3.3 Presenting the results

The results of a Batch analysis are best shown in graphical formatcomparing the life (or operating cost) of each of the batches fitted duringthe fitment period. It is of value to display the batches for one or moreprevious fitment periods as well.

While tyre life (preferably in kilometres, but acceptably in hours) is a key

measure of batch performance, the operating cost of the batch is usuallymore important as it accounts for both tyre purchase price and tyre life.Hence the Batch graph can be presented in terms of tyre life (higher isbetter) or in terms of tyre operating cost (lower is better).

Sample Batch Graphs are shown in Attachment 2.


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ATTACHMENT 1 Biased/Unbiased Life Calculation Sheet


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ATTACHMENT 2 Sample Batch Graphs

Case A: 37.00R57 tyres on Cat 789 trucks

Cat 789 haultruck tyre life

The above graph shows the tyre batch performance history (based onkilometre life) for a fleet of Cat 789 haultrucks fitted with 37.00R57 tyresfrom three manufacturers (Bridgestone, Goodyear and Michelin).

The Cat 789 fleet was introduced to this minesite in 1993 and the graphplots batch performance of the original equipment tyres and thereplacement tyres during six-monthly fitment periods (apart from thefirst) until September 1997.

Each coloured or shaded column represents a batch of tyres of a particularspecification as identified in the legend. The number of tyres in each batchis indicated at the top of the column. An F or R indicates that all tyresof that batch were fitted new to the front or rear axle respectively.

Comments: A number of batches contain less than 12 tyres and hence the

calculated average life for each of these batches should be treated asnot being statistically reliable. This indicates a fall-down inplanning during the fitment stage (or that an insufficient quantityof tyres was purchased initially for testing).


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Case B: 33.00R57 tyres on Cat 785 trucks

The two graphs below show the tyre batch performance history (based onkilometre life and cost per hour, respectively) for a fleet of Cat 785haultrucks fitted with 33.00R51 tyres from three manufacturers

(Bridgestone, Goodyear and Michelin).

0

5,000

10,000

15,000

20,000

25,000

30,000

Cat 785 O.E. tyres Fitments to Mar 90 Apr 90-Sep 90 Oct 90-Mar 91

Mi XHD1 A BS VRLSA E2A GY RL-4H 4S

Km

Batch Fitment Period

Batch LifeNumber above column notes batch size (no. of tyres).

25 158

28

14

29

19

Cat 785 haultruck tyre life

$0.00

$1.00

$2.00

$3.00

$4.00

$5.00

$6.00

$7.00

$8.00

$9.00$10.00

Cat 785 O.E. tyres Fitments to Mar 90 Apr 90-Sep 90 Oct 90-Mar 91

Mi XHD1 A BS VRLSA E2 GY RL-4H 4S Av. fleet spee

Batch Fitment Period

Cost/tyre hr Km/hr

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.010.0

Cat 785 haultruck fleet speed & tyre cost/hour

The Cat 789 fleet was introduced to this minesite in 1989 and the graphsplot batch performance of the original equipment tyres and thereplacement tyres during six-monthly fitment periods (apart from the

first) until September 1990.


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40.00R57 XDR B) is noted at the base of the column and the number oftyres in the batch at the top of the column.

The graph shows, in fact, three distinct applications:

Tyres fitted new to the front of Kom 830 trucks Tyres fitted new to the front of Cat 793 trucks Tyres fitted new to the rear of either truck type.

Comments: A number of batches contain less than 12 tyres and hence the

calculated average life for each of these batches should be treated asnot being statistically reliable. This indicates a fall-down inplanning during the fitment stage (or that an insufficient quantityof tyres was purchased initially for testing).

Notwithstanding the above comment, the graph shows someinteresting trends, e.g.

o The Michelin XDR tyre (in the relevant compound) is the bestperforming specification in each application, outperformingthe other Michelin specification (which was an XKD1) andthe Bridgestone (VELS) tyre.

o Tyres fitted new to the front axle of Cat 793 trucks had ahigher average scrap life than the same specification tyresfitted new to the front axle of Kom 830 trucks.

o Tyres fitted new to the rear axle of Kom 830 or Cat 793trucks had a higher average life than the same specification

tyres fitted new to the front axle of either truck.

The above graph was plotted for a very large mine with a large fleet of220mt trucks incorporating different truck types, with tyre fitments beingmixed between the two truck types. Batch analysis is typically muchsimpler when a single truck type is involved as was shown in Case A.

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