A product specification is the document that sets out what a product has to do, the conditions it has to do it in, and the limits on cost, size, mass and timing. Engineers design to it and suppliers quote against it; later, it is what a prototype is tested against. It should describe the result you need and leave the design that achieves it to the people developing the product.
The test of a good specification is whether two engineers, reading it separately, would agree on whether a given design meets it. That needs numbers with units, the conditions those numbers apply under, and a clear split between what is mandatory and what is preferred. A list of adjectives such as strong, light, quiet and easy to use gives each reader a different target, and the disagreement tends to surface months later at the first prototype.
What a product specification should contain
Headings vary by industry. A specification for a medical device looks different from one for quarry equipment, and a consumer product needs more on appearance than a test rig does. The ground that most good specifications cover is much the same, though.
Purpose, users and scope
Open with a short statement of what the product is for, who uses it and where. Then draw the boundaries: whether the document covers the whole product or one module, whether packaging, chargers and software are in scope, and which existing products it replaces or must work alongside. Use cases help. ‘A technician carries it up a ladder and hangs it from a cable tray’ tells a designer more than a paragraph of general description.
Functions
List what the product does as verbs: lifts, seals, measures, dispenses, locks. Separate the primary functions from the secondary ones. Written this way, the function list keeps the specification about behaviour and stops it drifting into a description of a mechanism someone has already pictured.
Performance targets
For each function, state how well it must be done, as a flow rate, force, speed, accuracy, run time or capacity. Give a target and, where it matters, a limit. A pump that must deliver at least 20 litres a minute is a different design problem from one that must deliver 20 ± 1 litres a minute. Record the reason behind each target where there is one. When two requirements conflict later, the reasons are what let you decide which one gives way.
Loads and environment
Specifications are most often thin here, and late redesigns often start here. Describe the forces the product sees in normal use, in foreseeable misuse and in transport, and how often each occurs. Then describe its surroundings: operating and storage temperatures, humidity, water and dust (an IP rating to IEC 60529 is the usual shorthand), sunlight, salt or chemical contact, vibration, shock and drops. If the product will travel by sea, sit in an unheated van over winter or be hosed down at the end of every shift, the specification should say so.
Interfaces
Interfaces are the boundaries with everything the product touches. They include mounting points and fixings, mating parts made by others, electrical connectors and supply voltages, fluid connections and thread forms, communication protocols, and the human interface of handles, buttons and displays. Where an interface is fixed, give the controlling dimensions and refer to the drawing or standard that defines it. A hole pattern that has to match an existing customer chassis is a hard constraint and should be marked as one.
Life and maintenance
State the service life in a unit that suits the product: years, operating hours, cycles or distance travelled. Say what maintenance is acceptable, how often, and which parts are expected to be replaced. Ten years means little for a hinge until you add how many times a day it opens.
Cost, volume and timing
A unit cost target needs the volume it applies to. A part that is reasonable to machine at 200 a year may be far too expensive at 20,000, where a moulded or pressed version costs much less per part once its tooling is paid for. Give the annual volume, typical batch sizes, the expected ramp after launch and the date the first saleable units are needed. If there is a ceiling on tooling investment, state it, because it can rule out a process before any design work starts.
Regulatory context
List the markets the product will be sold in and the rules that apply there. That usually means product safety legislation for machinery, electrical equipment, pressure equipment or medical devices, restrictions on substances such as RoHS and REACH, marking and labelling, and any industry or customer standard the buyer will expect. The specification does not need to interpret every clause. It should name each document and its edition, so the design team works to the right one from the start.
Manufacturing and supply constraints
If production has to use your own factory, an existing supplier or a particular process, record it. The same goes for preferred materials, finishes and colours, and for any bought-in parts that must come from a preferred supplier list.
Measurable and vague requirements
A requirement is useful when it has a quantity, a unit, the condition it applies under and, ideally, the method that will be used to measure it. The table sets common vague requirements beside versions a design team can work to. The numbers are illustrations; yours will come from the product and its market.
| Vague requirement | Measurable version |
|---|---|
| Lightweight | Mass no more than 2.5 kg including battery and strap |
| Waterproof | IP67: dust-tight, and no harmful water entry after immersion at 1 m for 30 minutes |
| Quiet | Sound pressure level no more than 55 dB(A) at 1 m in normal operation |
| Works outdoors | Operates from −20 °C to +45 °C in direct sun; stored from −30 °C to +60 °C |
| Easy to install | Fitted by one person with standard hand tools, using no more than four fixings |
| Durable latch | 50,000 open and close cycles, with latch force staying between 20 N and 40 N |
| Low cost | A stated unit cost at 5,000 units a year, with tooling quoted separately |
Two habits make a specification much easier to use. The first is one requirement per statement, so each can be met or missed on its own; ‘light and waterproof’ on one line is two requirements sharing a number. The second is to mark each requirement as mandatory or desirable and to rank the desirable ones. When a concept meets every mandatory requirement and only some of the desirable ones, the ranking tells you whether it is still the right concept. A reference number and a one-line reason for each requirement take minutes to add and save long arguments later.
What to leave open
A specification that fixes the material and the mechanism has become a design, and usually an unexamined one. Leave open anything that is a means to an end: whether a housing is moulded or machined, whether a clamp is cam or screw operated, whether a frame is welded or bolted, which motor supplier is used. Fix those choices only where something outside the design requires it, such as an existing tool, a supplier agreement, a customer drawing or a regulation. When you do fix one, write down why. Someone will ask in six months, and by then the reason may no longer apply.
Where you are tempted to name a solution, state the property you need instead. ‘Stainless steel body’ becomes ‘no visible corrosion after 500 hours of neutral salt spray to ISO 9227’, which a coated aluminium housing or a moulded polymer might also meet, possibly for less.
Turning ‘must be strong’ into a load case
A portable instrument case has a moulded carrying handle, and the first draft of its specification says the handle ‘must be strong’. Packed with accessories, the case weighs up to 22 kg, so its weight is 22 × 9.81 ≈ 216 N. Working from that figure, the requirement becomes five statements:
- Working load. The case is lifted many times a day, often with a jerk as it leaves the floor. A dynamic allowance of 1.5 on weight gives a working load of about 324 N, applied vertically at the centre of the grip.
- Life. Twenty lifts a day on 250 working days a year for five years is 25,000 lifts. To cover heavier users, the specification asks for 50,000 cycles at the working load with no cracking and no loosening of the fixings.
- Misuse. People stand on handles, use them as tie-down points and carry two cases stacked. The specification sets a proof load of four times the case weight, about 863 N, held for one minute, with no fracture and a permanent set below 1 mm once the load is removed. If a product standard for the equipment sets its own handle test, that figure governs.
- Side load. A case tipped over or dragged by its handle loads it at an angle. A lateral load of half the weight, about 108 N, covers that.
- Temperature. The case is used from −20 °C to +50 °C. Many polymers lose impact strength in the cold and creep under sustained load in the heat, so the load cases apply at both ends of the range.
The designer now has loads to size the handle and its fixings against, and a test house has a procedure to follow. The requirement still says nothing about material, shape or how the handle attaches, which keeps those decisions with concept and detail design. There, the working and proof loads become inputs to a stress analysis and the cycle count feeds a fatigue analysis. Our guide to the factor of safety covers how margins are applied to loads like these.
Common gaps in a first specification
- Transport and storage conditions left out, though a pallet on a lorry or in a shipping container can see more vibration and wider temperatures than the product does in use.
- Cost targets with no volume attached.
- Interfaces described in words when a drawing of the mating part already exists.
- Requirements copied from a previous product without checking whether they still apply.
- No named owner, so changes agreed in meetings never reach the document.
A specification will change as the programme learns more, and that is healthy. Keep it as the one place where the current targets live, and make sure designers and suppliers are reading the same version.
How SixGate™ can help
You may have a clear picture of the product and a short list of requirements, or a customer’s tender document to respond to. SixGate™’s Specification stage turns either into a specification the rest of the programme can design to. The product, its timing, its performance targets and its cost are defined with you, along with what the manufacturing process needs if production is to succeed. You come away with loads, environment and interfaces stated in numbers, and a clear record of what is fixed and what is left open for design. The stage can be bought on its own or as the start of a full programme through to production.