Sizing a Job From Five Numbers
Most small mechanical jobs turn on a handful of figures, and having them before you start is the difference between one trip to the supplier and three.
1. What will it weigh?
Volume times density, from the material weight calculator. The comparison worth carrying: steel is 2.9 times aluminium (7.85 against 2.70 g/cm³), stainless is slightly heavier than plain steel at 8.00, and titanium is 57% of steel rather than a tenth of it.
That answers the questions that are expensive later: whether one person can move it, whether the mount will hold it, and what shipping costs.
2. What fastener, and what does it actually carry?
The thread reference gives nominal diameter, coarse and fine pitch, tap drill and — the number that matters — tensile stress area. An M10×1.5 carries load on 57.99 mm², not the 78.54 mm² its shank suggests. Between a fifth and nearly a third of the apparent section is thread.
3. How tight?
The bolt torque calculator works from preload, not from a table. That M10 in class 8.8 at 75% of proof is 25,225 N of clamp force, which needs 50.45 N·m dry — and only 30.27 N·m if the threads are waxed.
Note the condition, always. Applying a dry figure to a lubricated bolt overshoots the preload badly, and it is the most common way a correctly-followed torque number produces a wrong joint.
4. What ratio, and what speed?
For a drive, the gear ratio and belt and pulley calculators trade speed for torque exactly: 20T into 60T at 1,500 rpm gives 500 rpm and triples the torque. A 100 mm pulley driving a 250 mm at 1,450 rpm gives 580 rpm, a belt speed of 7.59 m/s and a belt length of 1,759.2 mm for 600 mm centres.
Belt speed is the figure people skip and the one that decides whether a belt is running inside its rating.
5. What force, and in which direction?
The hydraulic cylinder calculator distinguishes the two strokes, which the datasheet headline often does not. An 80 mm bore with a 40 mm rod at 160 bar pushes at 80.42 kN and pulls at 60.32 kN — 75%. Size on whichever stroke does the work.
Order of operations
Weight and material first, because they constrain everything downstream. Fastener size and its real load area next. Torque last, because it depends on both the fastener and the condition it will be assembled in. Drive ratios and cylinder forces sit alongside as the motion questions.
Written on one page before the job starts, those five figures answer most of what comes up — and they are all cheap to compute and expensive to discover halfway through.
What none of them cover
All of these are educational estimates that ignore losses, real-world variation and the specifics of your components. None of them is a substitute for a manufacturer's specification or a designing engineer's figures — particularly for anything structural or safety-critical, anything holding a load above a person, or any pressurised system.
Hydraulic work in particular carries hazards a force figure says nothing about: stored energy after shutdown, fluid injection injuries from pinhole leaks, and loads that fall when a line fails. Those belong with trained people and the equipment's own procedures.
Where the numbers come from matters
One structural detail worth knowing about these tools: the tensile stress area behind the torque calculation and the one in the thread reference are the same derivation, not two tables that happen to agree. An M10×1.5 is 57.99 mm² on both pages, and it stays that way because neither is a transcription of the other.
That is a small thing and it is the kind of small thing that goes wrong quietly. A reference table and a calculator that drift apart give two different answers to the same question, and the person using them has no way to tell which is right.
Recompute when a variable changes
Most of these figures are stable for a job, with one exception: the torque depends on the condition the fastener is assembled in, and that changes more often than people expect. New plated bolts, a thread cleaned with solvent, an assembly lubricant used to prevent seizing — each moves the nut factor and therefore the torque needed for the same clamp force.
If the condition changed after you looked the number up, the number changed too.
Keep the assumptions with the answer
The habit worth building is writing down the inputs alongside the result: not "50.45 N·m", but "M10×1.5, class 8.8, 75% of proof, dry, K = 0.2". A bare figure is unusable six months later and dangerous if applied to a different condition.
That is true of all five: a weight without the material, a ratio without the tooth counts, a cylinder force without the pressure and the stroke. The inputs are the part that lets someone else — or you, later — check whether the answer still applies.