ANSYS Nonlinear Functions Applied in Automotive Brake Pedal Design and Manufacture

Jessica Song GHSP

Shanghai, PRChina Randy Phinney

GHSP Grand Haven MI, USA

Abstract A finite element model for a unique automotive pedal beam production process was developed. Tooling and pedal beam design is improved through the simulation.

Introduction Conventional high volume pedal beams are manufactured using a progressive stamping operation. The nature of progressive stamping results in a relatively high material scrap rate for many pedal beam designs. The unique manufacturing process modeled here trims and bends a narrow strip of steel into a pedal beam thereby greatly reducing the material scrap rate.

The design of the bend tooling and bend geometry are our concerns. There are both geometric and strength requirements for the pedal beam. In addition, strength requirements of the bend tooling must be considered.

CAE tools find wide application in mechanical and electrical mechanism design. We simulate the whole bending process after stamping using ANSYS. The simulation is nonlinear, includes material nonlinearities, large deformation and contact. Figure 1 shows the bender bend heads and sweeps.

Figure 1. Bender Machine Bend Heads

Material The pedal is steel SAE950X. This is a very popular steel in structural automotive components due to it’s relatively high yield strength.

Modeling The model contains the trimmed steel strip to be bent; “hard” bend head and wiper, easy bend head and wiper. See figures 2 and 3. The steel strip is meshed with 3D solid elements 186. Contacts are defined between the bend heads and wipers. The bend heads and wipers are assumed to be rigid. The hard bend wiper can rotate about its axis. The material plasticity model is multilinear kinematic handening.

Figure 2. Steel strip, bend heads, and wipers

Figure 3. FEA model

The simulation contains contact, material property nonlinearity, large deformation, and large rotation. The component geometries are imported from CAD. The analysis is carried out on a DELL workstation with Windows 2000 Professional and ANSYS 8.0. The run time is about 6 hours.

The simulation includes two sequential bend simulations. These stages are referred to as “hard bend” and “easy bend”. “Hard bend” refers to bending the steel strip against it’s greatest moment of inertia. An “Easy bend” is normal to the hard bend and is about the steel strips least moment of inertia.

Bender Process The bender is a continuous manufacturing process. It consists of a series of machines which perform sequential operations transforming a steel strip into a formed pedal beam. 1. The first machine is the coil feeder. The coil feeder controls the coil as the steel strip is fed off into the

next station. 2. Straightener/Feeder. This unit straightens the steel strip and controls the axial motion of the steel strip

by pushing and reversing as required. 3. Trim die. This station is a “cookie cutter” which removes steel from the strip to form the pre-bend

outline of the pedal beam. 4. Cut-off Press. This station separates a completed beam from the continuous steel strip. The steel strip

is cut-off by retracting it back to the cut-off press after bending is complete. 5. Bending. This station is the subject of our analysis and consists of the “bend heads” and “wipers” for

both the easy and hard bends.

The Bending operates by extending a “bend head” to guide the steel strip during bending. There are 2 bend heads oriented at 90 degrees to each other, one for the “hard” bends and the other for the “easy” bends. The easy and hard bends are done sequentially. Typically, but not necessarily, the hard bend is done first. The total bend sequence may consist of as little as a single bend up to several easy and hard bends.

Simulation The continuous metal strip is modeled using solid elements and contact. The non-free end of the trimmed steel strip has a fixed boundary condition. This end of the strip is at a sufficient distance from the bend heads to be equivalent to the steel strip in the actual bender. Steps: 1. The metal strip is indexed to the correct axial position. 2. The correct bend head for the current bend is extended into position. 3. The wiper is rotated by a preset number of degrees. During this rotation the wiper will contact and

deform the trimmed steel strip. Note that the wiper is free to rotate about its axis. This rotation is only restrained by a weak spring.

4. The wiper rotation is reversed to its start position. 5. The bend head is retracted to its home position. 6. Steps 1. through 5. are repeated for all bends required to form the pedal beam. 7. The final bend geometry is examined and measured to see if it is correct. If not changes may be made

to the trimmed shape and sweep angles and the analysis repeated. 8. The final nodal locations of the pedal beam are used to create a solid model of the finished beam

geometry. An ANSYS APDL macro has been prepared to accomplish this geometry creation. This model is sent back to design engineering to be used in the final pedal beam assembly.

During the simulation the torques and contact forces on the sweeps and bend heads are calculated. Figure 4 and figure 5 are hand bend and the wiper torque. Figure 6 and figure 7 are easy bend and the wiper torque. Monitoring of these values ensures that the bender machine limits are not exceeded.

Figure 4. Hard Bend

Figure 5. Hard bend torque versus sweep angle

Figure 6. Easy Bend

Figure 7. Easy bend torque versus sweep angle

Results and Discussion The trimmed shape, bend head geometry, and wiper geometry determine the final shape of the pedal beam. Through the use of this analysis we can test the proposed design of the trim die, bend heads, and wipers to ensure that they can produce the correct pedal beam geometry.

Tooling and time spent in trial, test, and error is expensive. This simulation can greatly aid tooling design, reduce cost, and reduce time to market.

Potential problems with hard bend stability, sensitivity to edge conditions, tool wear, beam strain history, beam cracking, etc. can be detected and corrected before tooling design is finalized.

Future Work This ANSYS analysis may be “packaged” using APDL programming to create an end user software package. The non-analyst pedal beam designer enters the trim die geometry, bending data, etc. The software then returns the finished pedal beam geometry. This ensures the pedal beam design will be manufacturable on the bender equipment and what tooling will be required to make this design.

A further enhancement is to add required strength data to the end user inputs. A analysis of loading on the pre-strained beam may then be done to ensure that the strength and stiffness requirements are met.

Large cost savings may be achieved by using common tooling for the largest possible number of pedal beam designs. This software will enable the pedal beam designer to investigate how to generate pedal beams which meet new design requirements while using existing tooling. Further, new tooling may be designed using this software to ensure the new tooling is capable of producing as broad a family of pedal beams as possible.

Conclusions The nonlinear capabilities of ANSYS have allowed the accurate simulation of this novel manufacturing process. The analysis can guide our design and manufacturing, reduce cost, and shorten the product development cycle. Using APDL this analysis becomes an end user software tool to help designers evaluate designs and provide modification guidelines. The product quality is greatly improved and the cost is reduced.

References

1) Kenneth G. Budinski, Michadl K, Budinsk,Engineering Materials,Prentice Hall 2001, Upper Saddle River, New Jersey, Columbus, Ohio

2) ANSYS Structural Analysis Guide, Release 8.0, 2003, ANSYS, Inc.Canonsburg, PA