Ten Key Questions for Spare Parts Planning

1)      How does demand behave?  To answer this, you must ask yourself the following:

a) How often do you expect to receive a demand for a given spare part?

b) What is the expected magnitude of a demand transaction when it occurs?

c) Are  failures based on age, or use, or both?

d) How large is the installed base of a given spare part?

(Technical note:  Historical data on the time interval between demands (inter-demand interval) and on the order of magnitude of demand transactions (demand order sizes) can be used to estimate the likelihood of a demand occurring in a given time interval and its transaction size using statistical techniques such as Croston’s method or a compound Poisson distribution[i].  If failure of a part (and the subsequent need for replacement or repair) is time-dependent (as many are), then the combined use of a type of Erlang distribution to estimate the interval and a Poisson distribution to estimate the quantity may be more appropriate.[ii])

2)      Are some spare parts much more important than others?  Some of the key questions here include the following:

a)  How expensive is the item?

b)  What does it cost to store and transport the item?

c)  What are the consequences when the part fails?

d) Do the consequences of a failure compound with time?

Factors like these are used to determine a part’s “criticality”.  For more critical parts, you usually need to have more safety stock.  That buffer stock may need to be geographically distributed near potential sources of demand and/or expedited delivery may be necessary.

(Technical note:  Where the answer to “d.” above is “yes”, then the use of an Erlang distribution may be helpful to estimate the duration of the wait time for the customer.)

3)      Is demand affected by the conditions in which the part is used and/or the level of preventative maintenance that it receives?  In cases where the answer is “yes”, then the algorithms and statistical approaches that are used to calculate demand and inventory requirements may need to be tailored for different situations.

4)      Are the magnitude and sources of demand such that requirements can be modeled as a trend over time with appropriate adjustments for seasonality?  If so, then this simplifies the planning considerably in that the requirements for such a spare part may be able to be modeled in a way that is similar to non-spare parts.

5)      Is the supply network composed of a single stage or multiple echelons?  The calculations for safety stock are different for each structure.

6)      Are the failed parts scrapped (consumed) or repaired and used again?  Where parts to be replaced are repaired and used again, a determination must be made as to whether the failed part is beyond repair and should be scrapped.  When new replacement parts should be purchased must also be determined, and tracking by serial number is required.

7)      Are the purchase, or manufacturing, batch sizes significantly larger than the expected demand quantities in a period?  If so, then this should be taken into account when planning resupply and safety stock.

8)      Is the supply constrained by a budget?  If so, you should take this into consideration when planning supply as well.

9)      Will requirements be reviewed periodically or continuously?  Continuous (or nearly continuous) review systems are quite feasible with modern communications and computing technology, and in many, if not most cases, they can yield better results.  A continuous review system reevaluates supply requirements each time an actual demand is generated.  Where a large number of spare parts must be monitored by a limited number of planners, however, it may be more practical to periodically review requirements (once per inter-demand interval or some multiple/fraction of inter-demand interval) for non-critical items and plan safety stock to account for demand variability over lead-time and the review period as well as variability in lead-time for those non-critical items.  Critical items, particularly expensive ones, should probably be evaluated continuously.  It may be useful to segment spare parts by levels of criticality and treat each group of parts accordingly.

10)   How many time periods of inventory do I currently have on hand for each spare part and for each category of spare parts?

A fundamentally sound approach to spare parts management can be summarized as follows:

  1. Understand your demand patterns
  2. Classify your parts (e.g. by criticality, by demand pattern, and/or cost, etc.)
  3. Apply the appropriate forecasting model and statistics
  4. Employ an efficient algorithm to find inventory targets and purchase quantities that meets the specific needs, constraints, and goals of your business including the structure of your value network, requirements of your customers, and the costs and risks/uncertainties that you face.  Keep the approach as simple as possible within those conditions.

(Note:  Many formal statistical approaches require assumptions that may not hold in your business.  In most cases, a heuristic that searches for a high value solution that conforms to real-world constraints, leveraging statistical theory where appropriate, is the most useful approach. )

5.  Deploy this algorithm through an easy-to-use, fast, visual and interactive tool that functionally meets your specific requirements, but  doesn’t “break the bank”.

As we enter this weekend, I leave you with one more thought — this time, from Socrates:  “The wisest man is he who knows his own ignorance.

Have a wonderful weekend!


[i] Lengu, D., Syntetos, A., Babai, M., “Spare Parts Management:  A Distribution-based Approach”, Salford Business School Working Paper Series, 342/11.

[ii] Saidane, S., Babai, M., Aguir, S., Korbaa, O., “Spare Parts Inventory Systems under an Increasing Failure Rate Demand Interval Distribution”, Proceedings of the 41st International Conference on Computers and Industrial Engineering,  2011.

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About Arnold Mark Wells
Industry, software, and consulting background. I help companies do the things about which I write. If you think it might make sense to explore one of these topics for your organization, I would be delighted to hear from you. I am employed by Opalytics.

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