Lessons · Engineering · Spring rate, deflection and the force you get
Rate, deflection, force
A spring's rate is force per unit deflection, and for an ordinary spring it is constant: force equals rate times deflection. Rate is a property of the spring; force is what you get once you decide how far to compress it.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is the whole of spring selection and it is routinely confused, because catalogues list rate and designers want force. The two are only the same number if the deflection happens to be one.
How to think about it
Write down which two of rate, deflection and force you actually have, then solve for the third. Check the deflection you need is inside the spring's working range: a rate is only constant until the coils start to touch.
Worked example
Force = rate x deflection.The whole relationship.
Rate is a property of the spring, in N/mm.From the catalogue.
Force is what you choose by how far you compress it.Within the working range.
Beyond the working range the rate stops being constant.Coils bind and it stiffens sharply.
Your turn
A spring of rate 8 N/mm is compressed 15 mm. Write the force in newtons.
8 x 15 = N
Solve one, graded on the server
The trap
Reading a rate as a force. 8 N/mm is not eight newtons; it is eight newtons for every millimetre, and at fifteen millimetres it is a hundred and twenty.