Here Phillip Mccluskey from Camloc Motion Control outlines how gas springs should be specified. Part 2 tomorrow.
The primary function of a gas spring is to lift and support an object in a safe and controlled manner. For the designer and end-user of an application, the handling forces are the most important consideration; these are the human and mechanical interactions the spring has with an application.
Handling forces can be categorised as either a System or Ergonomic force, and can be related back to the P1-P4 chart (discussed in Technical Guide – Edition 1: Gas Spring Overview), seen in figure one.
The P1 and P4 points of the chart are System Forces which can be calculated and must be taken into consideration by the designer of the application.
P1 is the force required to keep the application fully open, e.g. a car tailgate being held in open position.
P4 is the maximum load the system will experience. The spring is reaching full closure in the compression direction, the bracket and the hinge design must be capable of withstanding this force.
The P2 and P3 points are Ergonomic Forces. These are the subjective forces that the end user will assess the gas springs performance by.
P2 is the key handling force when lifting a lid, it dictates when you have reached an in-balance position. In the example of the car tailgate, the tailgate is shut and the end user is opening the tailgate; the target for this is to be less than 60N.
P3 is the key handling force when closing a lid, this is the force required to close when the spring is fully extended, the target for this is to be 60N or less.
Mounting Positions & Gas Spring Sizing
Mounting Points Overview
There are two mounting points for a gas spring, the ‘fixed’ and ‘moving’ mounting points.
As the names suggest, the fixed mounting point remains fixed, whereas the moving mounting point rotates through an arc as the application opens and closes.
As a rule of thumb when positioning a gas spring, Camloc Motion Control start with the moving mounting point approximately 1/3 the length of the lid from the hinge as shown in figure two.
This provides an extremely rough guide as to where to place a gas spring, but if this is developed further it will also give an indication of the size of the spring required.
Simple Moment Balance
If we begin by considering the application to be a simple moment balance without a gas spring involved, in basic mechanics terms the application can be considered as a second-class lever.
The application is pivoted at point A, the lid weighs 50kg (G) and the centre of mass is equi-distant at a length of L between pivot A and somebody holding the lid up at point B.
To calculate the upward force (F), an individual must apply to keep the lid horizontal and in balance.
Taking this a step further, if the centre of mass is moved further to the right (away from the pivot), no longer being equi-distant along the hatch; the centre of gravity (XG) is now 0.8m from point A, with the total length of the hatch (Z) being 1.2m.
Both solutions are simple because all forces are perpendicular to the beam and there is no gas spring involved. By including a gas spring, the problem becomes more complex.
Simplified Gas Spring Application
For the remainder of this section, we will refer to Figure 5 and list of terms:
Z = Length of the Lid (m)
XG = Centre of Gravity (m)
LS = Radius of Gas Spring Force (m)
F1 = Opening Force (N)
G = Mass of the Lid (N)
F2 = Closing Force (N)
n = Number of Gas Springs
Including a gas spring to the earlier example, the size of the gas spring required can be estimated using the following (simplified) formulas. These use the principle of the line of action of a force.
To calculate the forces involved, two terms in particular should be taken into consideration:
LS – Radius of the gas spring force (m), this is the distance between point A and the centre line of the gas spring.
n – Number of gas springs per application (normally two).
The formulas are used to give an estimate of the minimum required force for a specified mounting geometry.
They will not provide optimum mounting positions and a better solution may be found using specialist software.
2.04 Opening Force Calculation
The force required to hold the lid open (F1) can be calculated using the formula:
Gas Spring Force (F1) = Mass of lid x Centre of Gravity / Radius of Force x Number of Springs.
Closing Force Calculation
Similarly, the force required to close the lid (F2) can be calculated using the formula:
Closing Force F2 = Number of Springs x Spring Force F1 x Radius of Force / Lid Length
Calculation of the opening and closing forces can be achieved using the values below:
Opening Force (F1)
F1 = (50 x 9.81) x 0.8 / 0.25 x 2
F1 = 784.8N
Closing Force (F2)
F2 = 2 x 784.8 x 0.25 / 1.2
F2 = 327N
Changing the Moving Mounting Point
What happens to the forces when the gas spring mounting position is altered? Assuming the above example is the ‘mid-point’ and the radius of the gas spring force is moved either side of this by 50mm:
Moving the moving mounting point 50mm towards the hinge (i.e. LS becomes 0.2m):
F1 = 981N (+197N increase)
F2 = 327N (no change)
Moving it 50mm further away from the hinge (i.e. LS becomes 0.3m)
F1 = 523.2N (-261N decrease)
F2 = 523.2 N (+196N increase)
The calculations show that the closer towards the hinge the moving mounting point gets, the higher the opening force required. Whereas, moving it further away the opening force reduces, but the closing force increases.
Changing the Centre of Gravity
If the centre of gravity moves nearer or further from the hinge, this will also affect the opening and closing forces.
Moving the centre of gravity 0.2m towards the hinge (i.e. XG becomes 0.6m)
F1 = 588N (-196N decrease)
F2 = 245N (-82N decrease)
Moving it further away from the hinge (i.e. XG becomes 1.0m)
F1 = 981N (+197N increase)
F2 = 408N (+81N increase)