AirSpring

Air springs are commonly used in trucks, buses, and rail vehicles because their internal air pressure can be increased or decreased to adjust for the load carried while maintaining ride height.

Typically a valve on the chassis is mechanically linked to an axle and if the ride height is too low the value opens to admit high pressure air to the spring from a compressor, until the ride height increases enough to close the value. Conversely if the ride height is too high the value vents air from the spring to atmosphere until the ride height decreases enough to close the value. The value has a dead band so that typical suspension ride and roll motions do not open the valve.

In some cases, air springs are used as actuators to lower and raise an axle and tires. When a truck is heavily loaded, the air spring is inflated to push the axle's tires into the road and spread the load over a greater area of road.

In other cases, air springs may connect with each other or to a reservoir. The additional volume of a reservoir, for example, decreases the spring rate since for a change in ride height the incremental change in volume as fraction of the total volume is smaller. When springs connect to one another or to a reservoir, then the resistance to flow between the springs or the spring and reservoir may be tuned to give a frequency dependent response.

Parameters

Property File
The AirSpring properties are stored in a TeimOrbit format property file containing a table of the spring force versus spring height for different static pressures.
Trim Height
Determines the initial trim height value of the AirSpring. With the given trim load and trim height, MotionSolve determines the static internal spring pressure from the data in the air spring property file.
Trim Load
Force applied by air spring when the spring height is the trim height. MotionSolve determines the static internal air spring pressure that yields the trim load at the trim height according to the data in the air spring property file.
Use Bump Stop
It is possible to include an internal bump stop to limit the AirSpring ride height. For more information, see the Bump Stop documentation.
Scale
The scale options enable the scaling of the force and spring height without the need to directly modify the property file.
Note: The trim load and trim height are not scaled.

Connecting an AirSpring

The AirSpring exerts a force along a line between two bodies, it can be created as single or pair entities. When adding a AirSpring the guidebar appears showing the body and points collectors, and the Create Pair option.
  1. 1. From the guidebar, select the first body to connect.
    • Click Body 1 and select a body from the modeling window.

      OR

    • Click the Body 1 Advanced Selector and select the required body from the dialog.
  2. Similarly, select the second body to connect by clicking the Body 2 input collector.
  3. Select a point and enter the location where the spring connects to Body 1.
    • Click Point on Body 1 and select a point from the modeling window.

      OR

    • Click the Point on Body 1 Advanced Selector and select the required point from the dialog.
  4. Similarly, select the second point.
    Typically, air springs act between the chassis and an axle. Point 1 on Body 1 defines the top of the air spring, while Point 2 on Body 2 defines the bottom of the air spring.


    Figure 1. AirSpring connection points
  5. Open the AirSpring Entity Editor to edit the parameters.

Property File for AirSpring

The AirSpring properties are stored in a TeimOrbit format property file containing a table of the spring force vs spring height for different static pressures. When the model is submitted to the solver, MotionSolve reads the air spring properties from the TeimOrbit file for use during the simulation. If the units specified in air spring property file differ from the model, MotionSolve converts the air spring properties to model units, however it leaves the property file unchanged.

An example of the AirSpring property file is shown below:
$---------------------------------------------------------------------ALTAIR_HEADER
[ALTAIR_HEADER]
FILE_TYPE     	=  'asg'
FILE_VERSION  	=  1.0
FILE_FORMAT   	=  'ASCII'
$--------------------------------------------------------------------------UNITS
[UNITS]
(BASE)
{length    force     angle       mass    time}
'm'       'newton'  'radians'    'kg'    'sec'
$--------------------------------------------------------------Airspring Properties
[PARAMETERS]
inner_diameter     	= .100
bag_diameter       	= .200
spring_height      	= .460
cylinder_height    	= .250
outer_diameter     	= .200
meniscus_height    	= .153
$-------------------------------------------------------------------------AIRSPRING
[AIRSPRING]
(Z_DATA)
{pressure}
137875  	$ p0
275790  	$ p1
413685  	$ p2
551581  	$ p3
689476  	$ p4
(XY_DATA)
{spring_height <m>  force@pressure <Newton>}
.3048   	17792       	29358.26    	41368.46    	55602.77    	67612.96
.3302   	13344       	25799.68    	35585.77    	47595.97    	57826.88
.3556   	12455       	22241.10    	31137.55    	40033.99    	48930.43
.381     	9786.087    	17792.88    	26689.32    	35585.77    	44482.21
.4064   	8896.422    	16013.59    	23130.75    	31137.55    	40033.99
.4318   	6672.33     	13344.66    	21351.46    	27578.97    	35585.77
.4572   	6227.5      	12455.02    	18682.53    	24910.04    	31137.55
.4826   	5337.865    	11120.55    	16903.24    	22241.10    	28913.44
.508     	4448.22     	9341.26     	15123.95    	20461.81    	26689.32
.5334   	4003        	8896.44     	13344.66    	17792.88    	22241.10
.5588   	3558        	7561.97     	11565.37    	15568.77    	19572.17
.5842   	2668        	5782.68     	9786.08     	13344.66    	17792.88
.6096   	1779        	4448.22     	8006.79     	11565.37    	14234.30

Header Block

The HEADER block gives the file type, version, and format information.
$---------------------------------------------------------------------ALTAIR_HEADER
[ALTAIR_HEADER]
FILE_TYPE     	=  'asg'
FILE_VERSION  	=  1.0
FILE_FORMAT   	=  'ASCII'

Units Block

The UNITS block specifies the length, mass, force, time and angle units employed in the file. Units are not case sensitive – meter, Meter, METER, or Meter are all interpreted same. The UNITS block is required for all types of the data files to be read by the builder.
$--------------------------------------------------------------------------UNITS
[UNITS]
(BASE)
{length    force     angle       mass    time}
'm'       'newton'  'radians'    'kg'    'sec'

Parameters Block

The PARAMETERS block lists the values of inner diameter, bag diameter, spring height, cylinder height, outer diameter, and meniscus height of the rolling lobe spring in the AirSpring file. Rolling lobe springs are primary suspension springs in Trucks, Buses, Passenger Cars, Rail Vehicles, and other vehicles. To calculate the Torus Minor Radius and Torus Major Radius the following formula is used:
  • Torus Major Radius = Inner Diameter/2+(Bag Diameter-Inner Diameter)/4
  • Torus Minor Radius = (Bag Diameter-Inner Diameter)/4


Figure 2. AirSpring dimension parameters
$--------------------------------------------------------------Airspring Properties
[PARAMETERS]
inner_diameter     	= .100
bag_diameter       	= .200
spring_height      	= .460
cylinder_height    	= .250
outer_diameter     	= .200
meniscus_height    	= .153

AirSpring Block

The AirSpring block gives the spring force verses deflection for different static pressures. It has two sub-blocks:
  • Z_DATA
  • XY_DATA

The Z_DATA sub-block gives the nominal internal spring pressure for each force versus deflection curve. The XY_DATA sub-block contains multiple force versus deflection curves.

The first column of the XY_DATA sub-block is spring height and each subsequent column is the spring force for one pressure defined in the Z_DATA sub-block. Therefore the forces in the second column correspond to the first internal pressure (137875 Newton/Meter2), and the forces in the third column correspond to the second internal pressure (275790 Newton/Meter2), and so on.



Figure 3.

The spring height provided in the first column should cover the full range of the air spring heights. During a simulation if the spring height outside the range of height provided, MotionSolve linearly extrapolates the spring force with spring height.

Based on the trim load and trim height provided in the user interface, the Newton Raphson method is used to evaluate the equilibrium pressure from the spline data and subsequently, the force-displacement curve corresponding to the equilibrium pressure calculated is used for the simulation.

Outputs

The AirSpring outputs channels in the MotionSolve .plt, .abf, and .mrf files are summarized in the table below.
Type Component
Displacement Spring Trim Height
Velocity Spring Velocity
Force Spring Force