Compression Springs

Counting Coils

Counting the coils of a spring is one of the most frequently misinterpreted concepts of spring manufacturing. Failure to count and specify coils correctly can result in errors in specification drawings which can then carry through to the final production of the spring.

Each complete turn (360°) is one coil.

Diameters

There are four key points when measuring a spring

Wire diameter (d) – The diameter of the wire used to make the spring.

Outside diameter (OD) – the diameter of the spring from outside the wire.

Inside diameter (ID) – the diameter of the spring from inside the wire.

Mean diameter (D) – the distance from the centre of the wire to the centre of the wire on the opposite side of the spring. This cannot be measured by hand and only calculated. It can be done in one of two ways, ID + d or OD – d.

Measuring Length

The amount of deflection determines the spring length at any time.

Free Length is the spring’s length without any force or load applied.

Loaded Length is the length with a specific force or load applied.

Solid Length is the length at the maximum amount of deflection (i.e. when all of the coils are touching).

Gap & Pitch

The gap is the space between two adjacent coils, while the pitch is measured from the centre of the two adjacent coils. The simplest method of finding the pitch is to measure the gap between two adjacent coils and add the wire diameter.

Coil Winding Direction

A left-hand wound spring has the wire winding anti-clockwise.

A Right-hand wound spring has the wire winding clockwise.

If two compression springs are to operate together (one inside the other) it is important for them to be wound in opposite directions to prevent the coils from getting entangled.

Active & Dead Coils

Active coils are coils with spaces in-between each coil that can move under the force or load applied.

Dead coils are coils that are touching and do not move under the force or load.

End Types

There are 4 types of ends on a compression spring

  1. Open & unground
  2. Open & ground
  3. Closed & unground
  4. Closed & ground

The most common end type for a compression spring is closed & ground (4).

Outside Diameter Expansion

When a force or load is applied to a compression spring the outside diameter of the spring expands. This has to be taken into consideration if the spring is located in a container or recess. In addition to the outside diameter, tolerances need to be considered (tolerances are covered below).

Tolerances

Tolerances refer to the acceptable variation that allows a spring to function. Springs require larger tolerances than machine parts.

The tolerances are not always 0.15 (as shown in the video). Tolerances can vary depending on the size of the spring. Standard spring tolerances can be found in BS 1726.

Loads at Length

The free length of the spring is the spring’s length without any force or load applied. When a force or load is applied the spring is compressed and the reduction in length is known as the deflection. The amount of compression/deflection depends on the spring’s strength and the amount of force or load applied. The rate of compression/deflection is linear (i.e. at a constant rate).

It is advised to use a spring to 85% of its solid load or the spring’s working life may be shortened.

Spring Rate

Spring Rate is the strength of a spring as defined by Hooke’s Law, which states that the force (F) needed to extend or compress a spring by a given amount (s) is constant (i.e. as a load is applied the springs deflects at a constant rate).

Hooke’s Law is expressed as:

F = -k x s

‘F’ is the springs restoring force

‘k’ is the spring constant

‘s’ is the deflection

The negative sign in front of the spring’s constant means that the springs restoring force is opposite to the direction of compression.

Tension Springs

Counting Coils

Counting coils on a tension spring is the same as counting coils on a compression spring.

The type of ends on a spring will not affect the number of coils.

Diameters

There are three key points when measuring a spring

Wire diameter (d) – The diameter of the wire used to make the spring.

Outside diameter (OD) – the diameter of the spring from outside the wire.

Mean diameter (D) – the distance from the centre of the wire to the centre of the wire on the opposite side of the spring. This cannot be measured by hand and only calculated as D = OD – d.

It is not usual to measure the inside diameter of a tension spring.

Measuring Length

There are 4 kinds of lengths when measuring a tension spring

  1. Free length is the spring’s length inside the loops
  2. Free length – body only, is the length of the spring measuring the body of the spring only (i.e. coils)
  3. Free length – overall, is the length of the spring from outside the loops.
  4. Longest working length is the longest length the spring can stretch to without breaking or losing its initial tension.

Initial Tension

A tension spring already holds tension within the coils, this is an internal force that holds the coils tightly together. The external force needs to be more than the internal force for the coils to separate.

The example in the video shows the spring between 10N and 20N for the coils to be separated. Looking at the graph you can see the 10N is below the ‘Initial Tension’.

Loads at Length

The free length of the spring is the spring’s length without any force or load applied. When a force or load is applied the spring is deflected. The amount of extension/deflection depends on the spring’s strength and the amount of force or load applied. The rate of deflection is linear (i.e. at a constant rate).

If the load exceeds its elastic limit the spring will cause permanent deformation and will no longer work.

Spring Rate

Spring Rate is the strength of a spring as defined by Hooke’s Law, which states that the force (F) needed to extend or compress a spring by a given amount (s) is constant (i.e. as a load is applied the spring deflects at a constant rate).

Hooke’s Law is expressed as:

F = -k x s

‘F’ is the springs restoring force

‘k’ is the spring constant

‘s’ is the deflection

The negative sign in front of the spring’s constant means that the springs restoring force is opposite to the direction of extension.

Using a Vernier Caliper to Measure a Spring

Compression Spring

Divider

Using a digital vernier caliper to measure a compression spring.

Outside coil diameter – Place the external jaws across the outside diameter of the spring, making sure the jaws measure over a minimum of 3 coils (including the ends).

Inside coil diameter – Place the internal jaws inside the spring, make sure the jaws measure over a minimum of 1.5 coils.

Wire diameter – The wire diameter is measured in 2 planes, in line with the axis and 90 degrees to the axis. Use the mean of the 2 measurements to get the wire diameter (the wire diameters are slightly different in each axis due to deformation of the wire caused by the wire forming process).

Free length – Measure the full length of the spring without any force or load applied.

When using a vernier caliper do not exert excess force on the spring as this can cause deformation and will give a false reading on the vernier. It should feel like the jaws are only just touching the spring.

Tension Spring

Divider

Using a digital vernier caliper to measure a tension spring.

Outside coil diameter – Place the external jaws across the outside diameter of the spring, making sure the jaws measure over a minimum of 3 coils.

Inside coil diameter – It is not possible to get an accurate measurement for the inside diameter, this is due to the end loops been in the way.

Wire diameter – The wire diameter is measured in 2 planes, in line with the axis and 90 degrees to the axis. Use the mean of the 2 measurements to get the wire diameter (the wire diameters are slightly different in each axis due to deformation of the wire caused by the wire forming process).

Free length – Mesure the free length of a tension spring is done in three places; 1. inside the loops 2. the body of the spring its self 3. overall (outside the loops)

When using a vernier caliper do not exert excess force on the spring as this can cause deformation and will give a false reading on the vernier. It should feel like the jaws are only just touching the spring.