Why Faster Machines Do Not Always Increase Line Output

When output falls short, the obvious fix is a faster machine. The decision question is whether that machine will raise what the line ships, or only raise a number on a specification sheet.

The short answer: a line delivers what its slowest stage delivers in practice, not what its fastest machine can do on paper. A faster machine helps only if it is the stage that limits the line, or if it also improves how reliably that stage runs. Otherwise the extra speed shows up as waiting time.

A line is a chain

Take a typical packaging line: a bagger fills and seals packs, a checkweigher rejects under- and overweight packs, a case packer loads packs into cases, and a palletizer stacks the cases. Each stage hands product to the next. Over a full shift, no stage can pass on more than it receives, and no stage can ship more than the one behind it can accept.

That makes line output a minimum, not an average. If one stage can only process 36 good packs a minute over a shift, the line produces about 36 good packs a minute, however fast the others are rated.

What effective rate means

Nameplate speed is the rated speed under good conditions. The rate a stage actually delivers over planned time is lower. A common way to break the gap down is to multiply three factors, the components used in overall equipment effectiveness (OEE) measurement.

effective rate = rated speed x availability x performance x quality

availability = share of planned time the machine is actually running
performance  = actual speed while running / rated speed
quality      = share of output that is good on the first pass

In plain words: availability falls with breakdowns, waiting for material and changeovers. Performance falls with micro-stops and with running below rated speed to keep a fragile product or a sticky film under control. Quality falls with rejects, which are packs that consumed time and material but cannot be sold.

Worked example: four stages

Illustrative numbers, not a quote, benchmark or customer result.

Rated speeds are in packs per minute (the case packer in packs per minute equivalent). Each stage’s effective rate is rated speed times the three factors, counted in good packs.

Stage	Rated speed	Availability	Performance	Quality	Effective rate (good packs/min)
Bagger	60	0.90	0.95	0.98	50.27
Checkweigher	120	0.95	0.95	0.99	107.22
Case packer	50	0.80	0.90	0.99	35.64
Palletizer	80	0.90	0.95	1.00	68.40

The arithmetic for the case packer: 50 x 0.80 = 40.00; 40.00 x 0.90 = 36.00; 36.00 x 0.99 = 35.64. The bagger: 60 x 0.90 = 54.00; 54.00 x 0.95 = 51.30; 51.30 x 0.98 = 50.27 (rounded).

The case packer has the lowest effective rate, so it is the bottleneck. Notice that it is not the slowest on the nameplate by much (50 against the bagger’s 60), but its low availability and performance drag it well below the others. Line output is 35.64 good packs per minute.

Over an 8-hour planned shift (480 minutes), that is 35.64 x 480 = 17,107 good packs (17,107.2 before rounding).

This is a simplification. It treats each stage as if it were fed whenever it could run, and it ignores the way upstream rejects slightly lower what reaches downstream stages. For a first look at where the limit sits, it is usually good enough.

Two upgrade options compared

Option A: replace the bagger with a faster model rated at 80 packs per minute, with the same availability, performance and quality. Its effective rate becomes 80 x 0.90 x 0.95 x 0.98 = 67.03 good packs per minute. The case packer still delivers 35.64, so the line still delivers 35.64. Output per shift stays at 17,107 packs. The change in output is zero.

Option B: keep the bagger, and raise the case packer’s availability from 0.80 to 0.90 by fixing its most frequent stops. Its effective rate becomes 50 x 0.90 x 0.90 x 0.99 = 40.095, or 40.10 rounded. The bagger (50.27), checkweigher (107.22) and palletizer (68.40) all stay above it, so the case packer remains the limit and the line delivers 40.095 good packs per minute.

Scenario	Line output (good packs/min)	Output per 8-hour shift	Change
Baseline	35.64	17,107	none
A: bagger upgraded to 80/min	35.64	17,107	0
B: case packer availability 0.90	40.10	19,246	+2,139 (+12.5%)

Option B is 40.095 x 480 = 19,245.6 packs, which rounds to 19,246. The gain of +12.5% is the ratio of availability, 0.90 / 0.80 = 1.125. Improving the constraining stage raised output; the faster machine at a non-constraining stage raised nothing.

Buffers do not raise the limit

A buffer, such as an accumulation table or a small conveyor reservoir between two stages, lets the upstream stage keep running during a short stop downstream, and lets the downstream stage keep running during a short stop upstream. That protects the stages from each other and can lift availability a little.

A buffer cannot raise average output above the bottleneck’s average rate. If the case packer averages 35.64 good packs a minute, the buffer ahead of it either fills and the bagger waits, or it stays empty and never matters. Buffers fix stop-and-go, not capacity.

Changeovers and small batches

Every product change takes planned time out of running. A line that changes over four times a day for 30 minutes each loses two hours of an eight-hour shift to changeovers alone. The same line on one long run loses almost none.

Shorter runs therefore lower availability on every stage that needs adjusting, and the bottleneck stage sets how much it costs. The effect arrives in the cost of each pack through fewer good packs from the same wages and overhead. The arithmetic is set out in Cost per Good Pack.

What to measure before buying

Before approving a faster machine, collect a few weeks of real production data for each stage. A single good day or a vendor demonstration will not show the pattern.

• Run time per stage, as a share of planned time.
• Stop reasons, with duration, so frequent short stops are visible next to rare long ones.
• Changeover time, from last good pack of one product to first good pack of the next.
• Reject rate and where in the line rejects occur.
• Waiting time: starved (no product arriving) and blocked (nowhere to put product).

Starved and blocked time are the clearest clues. A stage that spends most of its stopped time starved is not the bottleneck. A stage that is rarely starved or blocked and always busy usually is.

When a faster machine does help

A faster machine raises line output in three situations.

• It is the bottleneck, and the next-slowest stage has headroom. Output rises until the next stage becomes the limit.
• It also improves availability or quality, for example through quicker changeovers, fewer jams or better rejection of defects, so the gain is not only in rated speed.
• Demand will grow and the rest of the line is being upgraded too, so the new machine will not become a stranded asset.

Whether the extra output is worth the purchase price depends on how many hours the equipment runs on real orders, which How Equipment Utilization Changes Automation Payback covers. The wider set of articles on this theme sits under Production Economics.

Keep reading

• Cost per Good Pack: A More Useful Measure Than Nameplate Speed: turns line output and changeover losses into a cost you can compare across scenarios.
• How Equipment Utilization Changes Automation Payback: shows how running hours change the payback of the same equipment.
• Automation Payback, ROI and TCO: What Each Metric Tells You: explains which metric answers which investment question.

Assumptions and limits

• Stage figures are illustrative. Your availability, performance and quality will differ by product, film, operator and maintenance practice.
• The model takes the minimum of stage effective rates. It ignores buffer size, upstream rejects reducing downstream flow, and speed that varies by product.
• Availability here covers all lost planned time, including changeovers. Some plants separate planned and unplanned losses, which changes the labels but not the product of the factors.
• Costs, financing, taxes, installation, ramp-up and residual value are not included. The example addresses output only.
• A move from one bottleneck to the next is assumed to be immediate. Real lines need tuning after any change.
• This is a comparison tool and does not replace engineering, safety, financial or legal review.