Search Results for design for assembly

A precipitation-hardened stainless steel poppet valve assembly used to shut off the flow of hydrazine fuel to an auxiliary power unit was found to leak. SEM and optical micrographs revealed that the final heat treatment designed for the AM-350 bellows material rendered the AM-355 poppet susceptible to intergranular corrosive attack (IGA) from a decontaminant containing hydroxy-acetic acid. This attack provided pathways for which fluid could leak across the sealing surface in the closed condition. It was concluded that the current design is flight worthy if the poppet valve assembly passes a preflight helium pressure test. However a future design should use the same material for the poppet and bellows so that the final heat treatment will produce an assembly not susceptible to IGA.

Components of a latch assembly used in a consumer safety restraint exhibited a relatively high failure rate. The failures were occurring after installation but prior to actual field use when failure could result in severe injury. Cracking occurred within retaining tabs used to secure a metal slide on an older design, whereas newer components showed no signs of failure. The latch assembly components were injection molded from an unfilled commercial grade of a polyacetal copolymer. Investigation of failed parts (including visual inspection, a specially designed proof load test, 59x SEM images, micro-FTIR in the ATR mode, and DSC/TGA/MFR analysis) showed no evidence of contamination or degradation from the molding process. The conclusion was that the parts failed via brittle fracture associated with stress overload. The stress overload was accompanied by severe apparent embrittlement resulting from a relatively high strain rate event and/or significant stress concentration. A relatively sharp corner formed by a retaining tab on the older design was shown to be a primary cause of the failures.

Catastrophic pitting corrosion occurred in type 304L stainless steel pipe flange assemblies in an industrial food processor. During regular service the pumped medium was pureed vegetables. In situ maintenance procedures included cleaning of the assemblies with a sodium hypochlorite solution. It was determined that the assemblies failed due to an austenite-martensite galvanic couple activated by a chlorine bearing electrolyte. The martensitic areas resulted from a transformation during cold-forming operations. Solution annealing after forming, revision of the design of the pipe flange assemblies to eliminate the forming operation, and removal of the source of chlorine were recommended.

The various methods of furnace, torch, induction, resistance, dip, and laser brazing are used to produce a wide range of highly reliable brazed assemblies. However, imperfections that can lead to braze failure may result if proper attention is not paid to the physical properties of the material, joint design, prebraze cleaning, brazing procedures, postbraze cleaning, and quality control. Factors that must be considered include brazeability of the base metals; joint design and fit-up; filler-metal selection; prebraze cleaning; brazing temperature, time, atmosphere, or flux; conditions of the faying surfaces; postbraze cleaning; and service conditions. This article focuses on the advantages, limitations, sources of failure, and anomalies resulting from the brazing process. It discusses the processes involved in the testing and inspection required of the braze joint or assembly.

In a helicopter engine connecting rod, high-cycle, low-stress fatigue fractures in bolts and arms progressed about 75% across the section before the final rupture. Factors involved were insufficient specified preload, inadequate tightening during assembly, and engine overspeed. The assigned main causes were design deficiency, improper maintenance during overhaul, and abnormal service operation. The problem can be solved by proper overhauling that ensures bolted assemblies are tightened evenly and accurately, in accordance with recommended torque values. Also, the manufacturer made various modifications, such as a thicker rod, fatigue resistant bolts, and more accurate preload measurements. The configuration of these rods were changed to a tongue-and-groove design to increase service life.

After the mixing machines were introduced into service, excessive assembly stresses and inappropriate detail design caused the premature failures of ice cream drink mixer blades shortly. The mixer blade is slightly deformed by the contact between the wavy washer at the bottom of the assembly and the bends at the bottom shoulders of the two mixer arms. Multiple fatigue crack origins on the inside radii of the bends at the bottom shoulders was revealed by analysis of the failed mixer blades. It was revealed by metallographic examination that the shoulders on the arms were cold bent, introducing tensile residual stresses on the inside radii of the shoulders and creating a localized area of fatigue susceptibility due to the inherent notch sensitivity of cold-formed 300 series stainless steel. It was established that the physical root cause was the design of the mixer blade. The addition of a stand-off washer between the wavy washer and the bottom shoulders of the blade or modification of the shape of the wavy washer to prevent contact with the blade shoulders was recommended.

A welded elbow assembly (AISI type 321 stainless steel, with components joined with ER347 stainless steel filler metal by gas tungsten arc welding) was part of a hydraulic-pump pressure line for a jet aircraft. The other end of the tube was attached to a flexible metal hose, which provided no support and offered no resistance to vibration. The line was leaking hydraulic fluid at the nut end of the elbow. Investigation supported the conclusion that failure was by fatigue cracking initiated from a notch at the root of the weld and was propagated by cyclic loading of the tubing as the result of vibration and inadequate support of the hose assembly. Recommendations included changing the joint design from a cylindrical lap joint to a square-groove butt joint. Also, an additional support was recommended for the hose assembly to minimize vibration at the elbow.

An air bottle, machined from a solid block of aluminum alloy 2219-T852, displayed liquid-penetrant crack indications after assembly welding. The air bottle was machined to rough shape, a 3.8 mm (0.15 in.) wall thickness cylindrical cup with a 19 mm (3/4 in.) wall thickness integral boss on one side. After annealing, hot spinning, annealing a second time, and tack welding a port fitting, the assembly was torch preheated to 120 to 150 deg C (250 to 300 deg F). The port fitting was then welded in place. Final full heat treatment to the T62 temper was followed by machining, testing, and inspection. The crack indications were found only on one side of the boss and on the lower portion of the hot-spun dome region. The metallographic specimens revealed triangular voids and severe intergranular cracks. The cracks displayed the glossy surfaces typical of melted and resolidified material. The localized cracks in the air bottle were from grain-boundary eutectic melting caused by local torch overheating used in preparation for assembly welding of a port fitting. A change in design was scheduled to semiautomatic welding without the use of preheating for the joining of the port fitting for the dome opening.

A series of poppet-valve stems fabricated from 17-4 PH (AISI type 630) stainless steel failed prematurely in service during the development of a large combustion assembly. The poppet valves were part of a scavenging system that evacuated the assembly after each combustion cycle. The function of the valve is to open and close a port; thus, the valve is subjected to both impact and tensile loading. Analysis (visual inspection, hardness testing, and stress analysis) supported the conclusions that the valve stems were impact loaded to stresses in excess of their yield strength. That they failed in the threaded portion also suggests a stress-concentration effect. Recommendations included changing the material spec to a higher-strength material with greater impact strength. In this case, it was recommended that the stems, despite any possible design changes, be manufactured from an alloy such as PH 13-8Mo, which can be processed to a yield strength of 1379 MPa (200 ksi), with impact energies of the order of 81 J (60 ft·lbf) at room temperature.

The governor on an aircraft engine failed and upon disassembly of the unit, it was discovered that the retainer for the flyweight pivot pins was broken. The channel-shaped retainer was made of 0.8 mm (0.030 in.) thick 1018 or 1020 steel. The part was plated with copper, which acted as a stop-off during carburizing of the offset, circular thrust-bearing surface surrounding the 16-mm (0.637-in.) diam hole. The bearing surface was case hardened to a depth of 0.05 to 0.1 mm (0.002 to 0.005 in.), then austempered to obtain a minimum hardness of 600 Knoop (1-kg, or 2.2-lb, load). Considerable vibration was created in the installation because of the design of the mechanical device used to transmit power to the governor. The pins were permitted to slide axially a small distance. Analysis (visual inspection, microscopic examination, and ductility measurements) supported the conclusion that failure of the retainer was the result of fatigue caused by vibration in the flyweight assembly. Impact of the pivot pins on the retainer also contributed to failure. Recommendations included redesign of the flyweight assembly, and replacement of the channel-shaped retainer with a spring-clip type of pin retainer.

In the EMD-2 Joint Directed Attack Munition (JDAM), the A357 aluminum alloy housing had been redesigned and cast via permanent mold casting, but did not meet the design strength requirements of the previous design. Mechanical tests on thick and thin sections of the forward housing assembly revealed tensile properties well below the allowable design values. Radiology and CT evaluations revealed no casting defects. Optical microscopy revealed porosity uniformly distributed throughout the casting on the order of 0.1 mm pore diam. Scanning electron microscopy revealed elongated pores, which indicated turbulent filling of the mold. Spherical pores would have indicated the melt had been improperly degassed. Based on these findings, it was recommended that the manufacturer analyze and redesign the gating system to eliminate the turbulent flow problem during the permanent mold casting process.

Two 38 mm (1.5 in.) diam threaded stud bolts that were part of a steel mold die assembly from a plastics molding operation were examined to determine their serviceability. Chemical analysis showed the material to be a plain carbon steel that approximated 1045. Visual examination revealed evidence of severe hammer blows to the clevis and boss areas and a gap between the die and the underside of the boss. Magnetic particle inspection showed cracks at the thread roots that, when examined metallographically, were found to contain MnS stringers. The cracking of the threads was attributed to a poor stud bolt design, which allowed a high stress concentration to occur at the base of the threads upon application of a lateral load. It was recommended that bolts of a new design that incorporated a stress-relieving groove be used. Threading of the bolt to eliminate the gap between the lower face of the boss and the die and an improved method of inserting or removing the bolt to avoid hammering (use of a wrench on a square or hexagonal boss) were also recommended.

This article provides background information needed by design engineers to create part designs optimized for plastics and plastic manufacturing processes. It describes the four essential elements of plastic part development, namely, material, process, tooling, and design, and provides general design rules for the plastic forming processes covered. It also discusses the steps involved in design validation and verification.

A heat transport pump in a heavy water reactor failed (exhibiting excessive vibration) during a restart following a brief interruption in coolant flow due to a faulty valve. The pump had developed a large crack across the entire length of a bearing journal. An investigation to establish the root cause of the failure included chemical and metallurgical analysis, scanning electron fractography, mechanical property testing, finite element analysis of the shrink fitted journal, and a design review of the assembly fits. The journal failure was attributed to corrosion fatigue. Corrective actions to make the journals less susceptible to future failures were implemented and the process by which they were developed is described.

A cylindrical spiral gear, part of a locomotive axle assembly, cracked ten days after it had been press-fit onto a shaft, after which it sat in place as other repairs were made. Workers at the locomotive shop reported hearing a sound, and upon inspecting the gear, found a crack extending radially from the bore to the surface of one of the tooth flanks. The crack runs the entire width of the bore, passing through an oil hole in the hub, across the spoke plate and out to the tip of one of the teeth. Design requirements call for the gear teeth to be carburized, while the remaining surfaces, protected by an anti-carburizing coating, stay unchanged. Based on extensive testing, including metallographic examination, microstructural analysis, microhardness testing, and spectroscopy, the oil hole was not protected as required, evidenced by the presence of a case layer. This oversight combined with the observation of intergranular fracture surfaces and the presence of secondary microcracks in the case layer point to hydrogen embrittlement as the primary cause of failure. It is likely that hydrogen absorption occurred during the gas carburizing process.

A ‘worn-out’ spiral bevel gear and pinion set was submitted for examination and evaluation. This was a spiral bevel drive set with the gear attached to a differential. The assembled unit was driving a new, large, experimental farm tractor in normal plowing and tilling operations. The primary failure was associated with the 4820H NiMo alloy steel pinion, and thus the gear was not examined. The mode of failure was rolling contact fatigue, and the cause of failure improper engineering design. The pattern of continual overload was restricted to a specific concentrated area situated diagonally across the profile of the loaded side, which was consistent on every tooth.