A machine that is running is not necessarily a machine that is producing. Between breakdowns, product line changes, micro-downtime, slowdowns, and rejected parts, a significant portion of production time is lost without showing up in the usual metrics.
The TRS, or synthetic rate of return, is specifically designed to highlight these losses. Known in English as OEE (Overall Equipment Effectiveness), it is one of the most widely used metrics in Lean and TPM initiatives. However, it is often miscalculated or misinterpreted. Definition, formula, numerical example, pitfalls to avoid, and levers for improvement: here are the essentials for turning it into a truly effective management tool.
What is TRS?
The TRS measures the proportion of time during which a piece of equipment actually produces good parts at its rated output rate, relative to the time during which it was expected to produce.
In France, the NF E60-182 standard defines the TRS and two related indicators, the TRG and the TRE. It is based on a breakdown of time into successive intervals:
- Total time : the entire calendar period.
- Opening Hours : the time during which the workshop is open.
- Time required : operating hours minus scheduled downtime (preventive maintenance, breaks, meetings, scheduled tests).
- Operating time : the required time minus unscheduled downtime (breakdowns, changeovers, adjustments).
- Net time : operating time minus downtime (micro-stops, slowdowns).
- Useful time : net time minus the time spent producing nonconforming parts.
OEE is the ratio of effective time to total time. A machine with 100% OEE would produce good parts, without any downtime and at full capacity, for the entire time it is expected to produce.
How to Calculate the TRS
The Three-Rate Formula
The TRS consists of three factors:
- Availability (D) = operating time / required time. It detects malfunctions, product changes, and adjustments.
- Performance (P) = actual production / theoretical production over operating time. It captures micro-stops and slowdowns.
- Quality (Q) = good parts / parts produced. It tracks rejects and rework.
TRS = D × P × Q
The Direct Formula
There is a simpler way to write this, which is useful for checking a calculation:
TRS = (number of good parts × theoretical cycle time) / required time
Both formulas yield the same result. The first is essential for determining where the losses occur; the second is the quickest to calculate.
Example of How to Calculate the TRS
Let's consider a machining center operating on an 8-hour shift, with a theoretical cycle time of 1 minute per part.
- Opening time: 480 min
- Downtime and scheduled maintenance: 60 min → Time required: 420 min
- Breakdown and change of series: 42 min → operating time: 378 min
- Units produced: 340
- Good coins: 330
Calculation:
- Availability = 378 / 420 = 90 %
- Performance = 340 / 378 = 90 % (The machine could have produced 378 parts in 378 minutes.)
- Quality = 330 / 340 = 97 %
- TRS = 0.90 × 0.90 × 0.97 ≈ 78,6 %
Verification using the direct formula: 330 good parts × 1 min / 420 min = 78.6%.
This result illustrates the multiplier effect: three rates that seem reasonable when considered separately result in a TRS that leads to more than an hour and a half of lost production time per workstation.
The Six Major Losses Measured by OEE
The TRS is based on the classification of the six major losses derived from the TPM. Each is associated with one of the three rates:
| Rate | Loss | Examples from a machine shop |
|---|---|---|
| Availability | Breakdowns | Spindle breakage, hydraulic failure, CNC alarm |
| Availability | Series Changes and Adjustments | Changing tools and chucks; adjusting the first parts |
| Performance | Micro-stops | Chip jam, sensor triggered, waiting for operator |
| Performance | Traffic Delays | Feed rate reduced as a precaution; cycle not optimized |
| Quality | Production Defects | Parts outside tolerance under steady-state conditions, rework |
| Quality | Start-up losses | Parts that were rejected while we were figuring out the right settings |
This matrix helps identify priorities: an OEE of 78% doesn't tell you what to do, whereas availability that's been compromised by excessively long setup times points directly to a specific area for improvement.
TRS, TRG, TRE, and TEEP: Don't Confuse Them
Several similar indicators exist, and mixing them up skews the comparisons:
- TRS : actual time / required time. It measures the equipment's efficiency based on the time it was supposed to be in production.
- TRG (overall efficiency ratio): productive time / operating time. It includes scheduled downtime.
- TRE (economic utilization rate): useful time / total time. It measures equipment utilization over the entire calendar period.
- TEEP (Total Effective Equipment Performance): the closest English equivalent to TRE, which indicates the maximum operational capacity.
To compare two machines or two sites, make sure they use the same metric and the same conventions.
How to Interpret the TRS
The 85% benchmark
85% is often cited as a world-class OEE in discrete manufacturing. This benchmark stems from the seminal work on TPM, which broke it down into 90% availability, 95% performance, and 99.9% quality. This figure must be put into perspective: a small-batch production shop with frequent changeovers does not face the same constraints as a line dedicated to a single product. The right target is the one you set based on your specific context and your own historical data.
The Most Common Pitfalls
Vague time-related agreements. Are breaks deducted from the required time? Is the planned changeover a scheduled shutdown or a loss of availability? Each choice affects the displayed OEE. The key is to define these rules once, document them, and apply them consistently.
A poorly chosen theoretical cycle time. If we use the observed cycle rather than the nominal cycle, performance appears artificially close to 100%, and the slowdowns disappear from the indicators.
Monitoring based solely on the overall TRS. An aggregated OEE hides imbalances. Two machines operating at 75% may have completely different problems. Always analyze D, P, and Q separately.
Forgetting the bottle neck. A TRS point gained on a bottleneck machine improves the capacity of the entire shop. The same point gained on an overcapacity machine contributes almost nothing.
A TRS used to evaluate teams. As soon as the TRS becomes a tool for individual assessment, reports of work stoppages become less reliable. It must remain a tool for understanding losses.
Improving OEE: Leverage Points by Component
The TRS is only valuable if it guides actions. Each component requires different levers.
Availability
SMED reduces changeover times. Preventive maintenance and root cause analysis (5 Whys, Ishikawa) address recurring failures. In machining, a significant portion of downtime often stems from setup: the time spent machining, measuring, and correcting the first few parts until a good part is produced.
Performance
Analyzing micro-stops requires precise data collection, as they are too short to be recorded manually. Optimizing cutting conditions and programs reduces slowdowns.
Quality
This is the aspect that is most often underestimated, even though it adds up to the other two: a rejected part has wasted machine time, material, and tooling for nothing. Statistical process control (SPC) makes it possible to detect deviations before they result in nonconformities.
Closed-loop automatic control affects both quality and availability. With Ellistat’s APC, the machine is automatically adjusted based on the measurements, ensuring that the first machined part meets specifications and stabilizing production on an ongoing basis. At one customer’s facility, the scrap rate dropped from 4% to 1.5%, resulting in an increase in OEE.
Measuring OEE in Real Time
An OEE calculated manually at the end of the week is used for reporting purposes. A continuously calculated OEE is used to take action. The automatic collection of machine status data, combined with the operator’s reporting of downtime causes, makes it possible to track the three rates in real time, identify deviations during the day, and consolidate losses by machine, by product, or by shift.
The TRS module of the Ellistat suite It centralizes this data, automatically calculates availability, performance, and quality, and cross-references it with quality data from the SPC and APC. This allows us to see not only how much we’re losing, but also why.
Conclusion
OEE is simple in principle but demanding to implement. When calculated correctly—with clear time conventions, a nominal cycle time, and a separate analysis of availability, performance, and quality—it becomes a common language between production, maintenance, and quality. Its value lies not in displaying a percentage, but in identifying where losses occur and focusing efforts where they yield the greatest returns.
FAQ
What is the formula for TRS?
OEE = Availability × Performance × Quality. It can also be calculated directly: OEE = (number of good parts × theoretical cycle time) / required time.
What is the difference between TRS and OEE?
It is the same metric. TRS is the French term, and OEE (Overall Equipment Effectiveness) is its English equivalent.
What is a good OEE?
The figure often cited is 85% in discrete manufacturing, but it must be adapted to the specific context: number of SKUs, batch sizes, and frequency of changes. The best goal is measured progress relative to your own historical performance.
What is the difference between TRS, TRG, and TRE?
They share the same numerator—effective time—but not the same denominator: the time required for TRS, the open time for TRG, and the total time for TRE. These definitions are specified in the NF E60-182 standard.
How can you quickly improve your OEE?
Start by identifying the component that is performing the worst. The quickest gains often come from reducing changeover and setup times (availability) and lowering start-up scrap (quality).
