Strokes per Minute vs Actual Tons per Shift
Every catalog quotes strokes per minute, and every yard measures tons per shift. The distance between those two numbers is where throughput expectations go to die — and the distance is not dishonesty, it is the difference between a capability and an operation. This article closes the gap: what the stroke rate assumes, what the operation takes out of it, and how to compute the tonnage your shift will actually deliver. The reference machine is the Q91-1000.
What the Stroke Rate Assumes
A strokes-per-minute figure is the machine’s cycle rate under ideal conditions: material staged and charged, the hold-down locking cleanly, the discharge clearing at the rate the box delivers, and the operator supervising rather than intervening. Under those conditions the cycle rate converts directly to cuts per hour, and cuts per hour multiplied by tons per cut gives the theoretical ceiling.
That ceiling is real, and no yard reaches it — because every assumption in the list above is an operational achievement rather than a machine property. The published tonnage bands exist precisely because we would rather quote the achievable figure than the theoretical one.
What the Operation Takes Out
Four deductions stand between the stroke rate and the shift’s tonnage, and all four are operational:
Loading. The excavator or grapple has to keep the tank charged, and its cycle time is rarely synchronized with the machine’s. Feeding in surges alternates between the ceiling and a stop; feeding in rhythm holds the middle. The difference over a shift is most of the distance between the bottom and top of any published band.
Discharge. Cut pieces have to leave the discharge zone before the next cut lands. A machine whose output banks up spends half its shift waiting, and the pile is the constraint rather than the machine. At 100 to 120 tons a day the discharge arrangement is a specification item, not an afterthought.
Material reality. The tons per cut is not a constant — a dense bundle and a single section are different weights, and the mix through the week moves the average. The band exists because the material moves it.
The maintenance window. Daily checks, weekly gap measurements and blade rotations are scheduled stops. At this class the window sits inside the production plan rather than against it, and the tonnage figure accounts for it — the plant guide covers the questions that expose whether the window is real.
Computing Your Shift’s Tonnage
The honest computation, run before the order rather than after:
- Start with the cycle rate — cuts per hour at the published figure.
- Multiply by your tons per cut — from your material mix, not from a brochure average. Weigh a week’s output if you can; estimate from the mix if you cannot.
- Apply the loading factor — realistically 0.7 to 0.85 of the ceiling, depending on how well your excavator keeps the tank charged.
- Apply the discharge factor — another deduction if your handling cannot clear at the box’s rate.
- Deduct the maintenance window — daily, weekly and rotation stops, in hours per week.
The result is the tonnage your shift will deliver, and it should land inside the published band for the class you are considering. If it lands below, the class is oversized for your operation; if it lands above, you are planning to run the machine at its ceiling, which is where blade life and cycle time go to die. The cycle-time guide covers what happens inside the cycle, and the full range publishes the bands per class.
The Two Ways Buyers Get This Wrong
The ceiling buyer. Computes from the stroke rate, plans the operation at the theoretical maximum, and discovers the gap in month one — usually blaming the machine. The gap is not the machine; it is the four deductions.
The pessimist buyer. Reads the bottom of the published band as the machine’s limit and sizes down a class. The band’s bottom assumes bulky mixed intake and a shared excavator; a yard with a dedicated grapple and sorted feed lands at the top, and the right class is the one the honest computation supports.
Both errors are avoided by the same computation, run honestly, before the order — and the quotation that comes back within two business days will confirm the class your numbers support.
The Deduction Table
The five deductions convert the stroke rate into the shift’s tonnage, and each has a defensible range. The loading factor runs 0.7 to 0.85 depending on how well the excavator keeps the tank charged — a dedicated grapple lands high, a shared excavator low. The discharge factor is a further deduction where the handling cannot clear at the box’s rate. The maintenance window is hours per week: daily checks, weekly gap measurements, blade rotations. And the tons per cut comes from your material mix, weighed if possible rather than estimated.
Written as a table with your numbers in it, the computation is defensible — and it is the document to bring to the quotation conversation, because it is the number your operation produces rather than the number a brochure promises. The band-reading guide covers the assumptions behind the published figures.
The Honest Computation
The computation protects against both failure modes. The ceiling buyer plans at the theoretical maximum and discovers the gap in month one, usually blaming the machine — the gap is the deductions, and they were visible before the order. The pessimist buyer reads the bottom of the band as the machine’s limit and sizes down a class — the band’s bottom assumes the worst version of every deduction, and a yard that manages them lands at the top.
Both errors are avoided by the same table, run honestly, before the order. Send your numbers with the inquiry and the written recommendation confirms the class whose band your computation lands inside — including when it is the class below the one you asked about.
The Computation, Documented
The five-step computation should be written down, because it is the document that makes the class decision reviewable. Cycle rate, tons per cut, loading factor, discharge factor, maintenance window — five numbers, each with its source: the published figures, your weighed output, your observed excavator behavior, your discharge observation, and your planned maintenance schedule. The total is your shift’s tonnage, and it is the number the class decision should be made against.
Written down, the computation also survives the second opinion. A supplier that reviews your numbers and confirms the class is describing a specification process; a supplier that reviews them and quotes a bigger machine is describing a sales target. Send the computation with your inquiry — the range table is the reference, and the written recommendation comes back within two business days.
The Computation as the Decision
The five-step computation is the class decision, and writing it down is what makes it reviewable — by you, by your second-in-command, and by the factory reviewing your inquiry. Five numbers with sources, one total, and a class whose band contains it with the heavy tail checked against the continuous rating. The computation replaces the two most expensive inputs to equipment decisions: tonnage pride and budget anxiety.
Bring the computation to the quotation conversation. The range table is the reference, and the written recommendation that comes back within two business days confirms the class — including when it is the class below the one you asked about.
Buyer FAQ
Why does no yard reach the stroke-rate ceiling?
Because the ceiling assumes ideal staging, synchronized loading, clearing discharge and no maintenance stops. Each assumption is an operational achievement, and the published bands quote the achievable figure instead.
What is the biggest deduction?
Usually loading — how well the excavator keeps the tank charged. Surge feeding alternates between the ceiling and a stop; rhythm feeding holds the middle where the tonnage lives.
How do I compute my shift’s tonnage?
Cycle rate times tons per cut, then apply the loading factor (0.7 to 0.85), the discharge factor, and deduct the maintenance window. The result should land inside the published band for the class.
What if my computation lands above the published band?
You are planning to run at the ceiling — which is where blade life and cycle time go to die. Either the class is oversized, or the assumptions behind the computation need a second look.
What if it lands below the band?
Check the deductions honestly — the band’s bottom assumes bulky mixed intake and a shared excavator. A yard with sorted feed and a dedicated grapple lands at the top, and the class follows the honest computation. Write the five-step computation down and bring it to the quotation conversation, because it is the number your operation produces rather than the number a brochure promises — and a supplier that reviews it honestly is describing a specification process. Write the five-step computation down and bring it to the quotation conversation — it is the number your operation produces rather than the number a brochure promises, and it is what the class decision should be made against.