Team 09F Senior Design Project

Sponsored by

C. J. Winter Machine Technologies, Inc.

234 Slide Arm Failure Analysis and Redesign

 

Home†††††††††† Introduction††††††††††† Team 09F†††††††††††††† Analysis of original††††††††††† Analysis of redesign ††††††††† Fatigue Testing†††††† Technical Article††††††††††††† Final Report††††††††††† Email

 

 

 

C. J. Winter 234 Slide Arm Redesign

Brent Vesa / Student, Team leader

Erik Hargrave / Team Member

Thanh Cao / Team Member

Dr. E. DeBartolo / Faculty Advisor

 

Abstract

The object of this project was to redesign a failed 234 slide arm within a 234SA rotary radial thread forming assembly.The assembly is produced by C. J. Winter as an attachment for Hydromat rotary transfer machines employed for thread forming operations.The 234 slide arm supports thread dies within the assembly and may be subjected to excessive stresses during thread forming.Finite Element Analysis (FEA) was performed on the original part revealing high stress areas.A new design was developed to reduce the stresses within the part.FEA analysis performed on the redesign indicates an improved factor of safety that will be supported by fatigue testing.

 

 

Nomenclature

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Figure 1. Slide arm nomenclature

 

 

Background

The 234 slide arm is a single part within a 234SA rotary radial threading attachment used to form threads on a Hydromat rotary transfer machine (see Figure 2).

 

Rotary threading attachment shown here

 

Figure 2 . Hydromat rotary transfer machine

 

d

 

c

 

b

 

a

 

a

 

Figure 3. 234SA rotary radial threading attachment

 

Within the attachment, two opposing slide arms (Fig. 3, a) hold a roller thread die (Fig. 3, b).Each die is aligned with the other within the 234SA by an alignment chain to prevent thread mismatch on the part.There is a thick outer ring surrounding the tool (Fig. 3, c).The ring contains two internal ramps oriented axially, and each ramp coincides with one of the two slide arms contained within the assembly.The outer radius of the slide arms is designed to clear the inner diameter of the outer ring.The entire 234SA attachment spins continuously during threading.As the outer ring is advanced axially by an air cylinder, the ramps press on the roller and pin.This forces the two slide arm heads, and therefore the roller dies, together around the unthreaded part stock (Fig. 3, d).The part stock is threaded as it is pinched between the two roller dies.Once the part is threaded, the air pressure is released, each slide arm returns to its starting position, and the threaded part stock moves on to the next station of the Hydromat machine.

The current 234 Slide arm is made from A2 tool steel that is cast, machined, and then heat-treated.From past experience, C. J. Winter has determined that this is the best manufacturing process.Machining the sliding surfaces, and drilling and tapping the holes remains necessary as finishing operations.C. J. Winter is satisfied with the performance of all other parts, so that the scope of the project is limited to slide arm redesign only.

A single C. J. Winter model 234 slide arm failed in service at a customer site.In an attempt to prevent future failures, C. J. Winter immediately contracted slide arm destructive testing to IMR test labs.Upon completion of the testing, C. J. Winter sought help from the Rochester Institute of Technology Mechanical Engineering department.The original slide arm factor of safety was calculated to be 0.85.This is an indication of why the original slide arm failed.

Previously, two production arms were sacrificed for mechanical destructive testing along with another slide arm that was completely machined from a block of steel.In addition, IMR labs designed a makeshift static testing fixture to fit an Instron frame with the intent of breaking the slide arms.The two cast arms failed, and the machined arm was disfigured, showing signs of imminent failure beyond the capabilities of the Instron frame.

 

Preliminary Analysis

The preliminary data, the three sacrificed arms, and a new production arm were presented to the team at an introductory meeting.A finite element analysis report of the current design was also presented.

The model was constrained to sliding motion along the ground sliding surfaces, and forces applied to the roller pin, thread die axle, and taper adjust screw as specified below (see Figure 4).††

 

Figure 4. Free Body Diagram

 

In the current design, force transmission to the slide arm is via three separate parts.The pinch force of thread rolling is achieved when the outer ring of the assembly is advanced by air pressure.The roller pin transmits the pinch force to the slide arm.The capability of the air cylinder responsible for advancing the outer ring, and the six degree ramps dictate that the maximum pinch force will be 7,000 lb.The roller is centrally located on the pin and the 7,000 lb force is evenly distributed with 3,500 lb on either roller pin hole on the outer radius of the slide arm.The opposing force is taken up by a spring between the two slide arms until the thread dies contact the unthreaded stock.Once the unthreaded stock is contacted, the spring force becomes insignificant, and the pinch force is opposed by the part being threaded and later by the incompressibility of the threaded stock (see Figure 4).

The reaction forces are transmitted from the stock through the thread dies and into the thread die axle.On one end, the thread die is adjacent to the axle hole at the free end of the slide arm.Approximately 75%, or 5250 lb, of the thread forming force is transmitted to the slide arm at this point.This percentage was determined by statically analyzing the thread die position on the die axle, and with a 7,000 lb pinch force.Within the slide arm body, the axle rests on the taper adjust screw, which is contained within the threaded blind hole.The remaining 25%, or 1750 lb, of the total force is transmitted from the die axle, to the taper adjust screw, and into the slide arm through the taper adjust screw.

 

 

 

 

 

 

 

 

 

 

 


Figure 5. FEA analysis of original design

 

For comparison purposes, the teamís FEA model was matched to the C. J. Winter FEA model by applying identical boundary conditions and mesh sizing.

 

Design

Finite Element Analysis was our roadmap to redesigning the slide arm.High stress areas of tension and compression were revealed and targeted as areas where additional material might increase the slide arm strength.Immediately the sharp corners and threaded holes were noted as stress concentration areas.The highest stresses occurred in compression surrounding the roller pin.The next highest stresses occurred in tension in the inner wall.Note that the original slide arm experiences 2.34x105 psi maximum stress under the operating load of 7,000 lb.

FEA of original slide arm revealed high stress areas at the outer wall location.Modifications consisted of strategic addition of material to the high-stress areas revealed during FEA analysis.The re-creation of the analysis opened up possibilities for exploration of the model in much greater detail.I-DEAS software allows for cross sectional analysis across a specified plane.One cross-sectional plane was placed along the threaded blind hole, which lies directly below the highest stressed pocket sidewall.Previously, our team had focused on the threaded blind hole as the original fracture point.FEA revealed that the highest tensile stresses occurred at the inner sidewall.Compressive stresses were noted, but were initially ignored since they were not likely to cause failure.

Torque is created by the mismatched opposing force planes, and need to be considered statically.The two force planes are separated by 0.78925 in, and the 7,000 lb force creates a moment of 5,524.75 lb-in (see Figure 4).Beam theory was applied in the pocket area to assist us while reducing the stresses within the arm during thread forming to determine the dynamic factor of safety due to bending fatigue.The static factor of safety was a quick and easy way to verify our improvement in slide arm design.The factor of safety was calculated using the formula Static factor of safety is equal to Yield Srength/ Maximum Von Mises Stress.The redesign factor of safety at 7,000 lb load is calculated to be 1.5, which exceeds the static requirement.

 

 

 

 

 

 

 

 

 

 


Figure 6. Proposed Redesign

 

 

Our redesign, including the addition of material, could occur in the existing configuration if the chain tensioner is reversed.Once reversed, the tensioners will have a tight clearance with the non-critical side of the slide arm pocket, and leave plenty of room for the addition of material to the critical side of the slide arm.Most corners on the existing part are near sharp, and only have radii due to the limitations of the casting process.Sharp corners cause stress concentrations, especially under near-failure conditions.There are two areas on the part that would benefit from the addition of a radius.The first opportunity occurs where the sliding surfaces meet the head of the part next to the die axle set screw.This area was given a radius of 0.08inch.The other obvious radius requirement occurs where the sprocket pocket meets the base of the slide arm head.The pocket in the slide arm serves two functions.First, at the upper end, there is clearance for a thread die.Since there are two thread dies used in the thread rolling process, and they must be aligned to prevent mismatch, a sprocket for the alignment chain is contained in the lower half of the pocket.Since this is the critical failure area, material must be added, and an adequate clearance must be provided for the alignment chain.This area was given a radius of 0.125 in.This radius allows for a significant decrease in stress concentrations, while maintaining an adequate clearance for the alignment chain.

The casting process was retained due its ability to create the necessary part geometry.A6 tool steel is the material of choice for its ability to be cast, machined, and heat treated.Since the material, production, and finishing process cannot be altered, strategic material addition remained the key to strengthening the slide arm.

The slide arm is a single part contained within a threading tool, so part-to-part clearances within the tool need to be considered. Along the sliding surfaces, the bases do not meet in the center, so there is a space left for the addition of material.The original design had a head depth of 0.9945 in.The new design has a head depth of 1.1195 in providing an additional 0.1250 in of material that was previously wasted space inside the tool.Along this same surface, the original slide arm had an angle that originated between the roller axle set screw and the sliding surfaces and tapered to the outer edge of the head.Both threaded holes are located in the base, and a large amount of material addition would strengthen the slide arm.The angle was eliminated, freeing up an additional 1/8inch.By doing this, we have also eliminated the need to maintain the angle tolerance, and the slide arm will be that much easier to manufacture.

The inner sidewall was the original fracture location, and required the addition of material.After consulting the glossy product literature about the location of parts within the threading tool, we noticed that the alignment chain tensioner has very little clearance with this side of the slide arm pocket.However, the other side of the slide arm has a huge clearance.We suggest that the part be mirrored to allow for additional material in the problem area without a clearance issue.One sidewall needs to remain thin due to the tight clearance of the chain tensioner.The tensile stresses in the outer sidewall were much less than the tensile stresses in the inner sidewall, and they remained within the acceptable limits set by the factor of safety calculations.The problematic inner sidewall was extended 0.230 in, therefore removing stress from both sidewalls.

Bending requires that a complimentary compressive stress oppose the pocket sidewall tensile stresses.This compressive stress obviously occurs on the slide arm outer radius, but is concentrated at the roller pin cutout directly behind the inner pocket sidewall.While compressive stresses are not as likely to cause failure, they must be considered.Since the pin is a press fit, there is no need to have two sides open.Neither the roller pin nor the roller have failed in service, and should never require removal.We suggest pressing the pin from the outer side, and filling in the inner recess.This decreases the compressive stresses dramatically, but moves the stress concentration to the other recess that opposes the non-critically stressed pocket sidewall.

 

Design Analysis

 

Figure 7. FEA analysis of redesign

 

The FEA results show that the maximum 2.34x105 psi stress of the original C. J. Winter design was reduced to 1.28x105 psi under the normal operating force of 7,000 lb.These stresses were compressive, and occurred at the recess for the roller The major difference in our proposed redesign is that the entire slide arm is mirror imaged.The sliding surfaces are opposite the original sliding surfaces.This is necessary for tensioner clearance, but should not affect the slide arm performance.The tensile stresses at the inner sidewall were shown to be 1.16x105psi.The corresponding factor of safety was calculated to be 1.55, which exceeds our static requirement.

 

 

 

Static factor of safety = Yield Srength/Maximum Von Mises Stress

 

C. J. Winter original slide arm factor of safety:

(This could be an indication of why the original slide arm failed.)

 

Redesign factor of safety at 7,000 lb load:

Exceeds the static requirement.

 

 

Testing

††††††††† For the purpose of fatigue testing, a fixture was designed and built following C. J. Winterís static testing fixture. The new fixture is different from the static testing fixture in that it has tabs allowing insertion into Instron grippers.

The team planned to test prototypes of the redesigned slide arm, but C. J. Winterís overseas casting vendor was unable to cast the prototypes within the projected schedule due to the current economic downturn. As a result, the team tested an original slide arm at RITís Center for Integrated Manufacturing Studies (CIMS) facility, where they operate a series 8100 Instron fatigue testing machine.The Instron fixture was set up to run cyclical testing from 1,200 lb to 7,000 lb in compression for one million cycles at 10 Hz.The original slide arm failed shortly after 2,000 cycles.

Evaluation of the failed slide arm supported the proposed redesign. Lines within the fractured surface, called beaches, indicated the source of the fatigue failure at the corner of the inner sidewall and sprocket pocket (see Figure 1).

The fixture created for fatigue testing, can be used in the future at CJ Winterís discretion to test the improved redesigned slide arms at a later date.

Acknowledgements

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The members of Team 09F would like to thank the following for their assistance with this project:

C. J. Winter

Mr. Liberato Pietrantoni

Mr. Sandro Belpanno

Mr. Paul Allart

Dr. Elizabeth DeBartolo

Dr. Alan Nye

Mr. Dave Hathaway

Dr. Michael Haselkorn

Dr. Kevin Kochersberger

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© 2003. Rochester Institute of Technology, Senior Design Team 09F. All rights reserved.

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