Raj S. Sodhi

Evaluating the unfastening effort in design for disassembly and serviceability

Disassembly is the process of physically separating a product into its parts or subassemblies. Recently, product designers are being challenged to address the concept of ‘ease of disassembly’ while configuring new designs. This is driven by the need for new products to undergo a design for disassembly and serviceability (DfDS) analysis. DfDS promotes design features and attributes, which reduce the subsequent disassembly costs. The disassembly process commonly involves an unfastening action. In this paper we present the unfastening effort analysis (U-effort) model, which helps designers to evaluate and select their fastener options. The U-effort model was developed from an experimental investigation of the most common fastener types used in industry. For each fastener type, the U-effort model identifies several causal attributes, and uses these to derive the U-effort index for a given case. From our experiments, we found that the most significant causal attributes are usually related to fastener size, shape or operational characteristics. The U-effort model is easily integrated into DfDS analysis schemes. The disassembly times generated from the U-effort model can be used to perform economic analysis of product service and/or end-of-life disassembly operations.

Kinematic synthesis of adjustable RRSS mechanisms for multi-phase motion generation

This paper presents a new technique for synthesizing RRSS mechanisms to achieve phases of coupler positions using the same hardware. This paper considers two- and three-phase problems with constant and adjustable crank and follower lengths. By specifying the joint axes of the RRSS mechanism, the constant length condition becomes the only design constraint. Coupler body positions are then selected with respect to the specified joint axes. These positions are incorporated in the constraint equation and the RRSS mechanism joint variables calculated.

Kinematic synthesis of adjustable moving pivot four-bar mechanisms for multi-phase motion generation

A four-bar linkage can satisfy up to five prescribed positions for the motion generation problem. The adjustable four-bar linkage, on the other hand, can satisfy more than five given positions by making some of the parameters adjustable. Limited work had been done in the area of motion generation problems of kinematic synthesis of adjustable four-bar linkages. This paper considers for the first time, the adjustment of a moving pivot. Equations are developed for the most complicated cases, which are two phase adjustable moving pivot problems with three positions in each of the two phases. Solutions are developed for two and three phase adjustable moving pivot problems. The numerical method developed for solving adjustable fixed pivot problems are extended to solve adjustable moving pivot problems. Synthesis examples are presented. Several Turbo Pascal programs are developed for solving the synthesis problems. Many user-defined AutoLISP functions and commands are specially designed for this work.

Kinematic synthesis of RRSS mechanisms for multi-phase motion generation with tolerances

Abstract This paper presents a new technique for synthesizing spatial RRSS mechanisms to achieve phases of both precise rigid body positions and rigid body positions with tolerances. This method is an extension of the adjustable RRSS motion generation synthesis method developed by the authors [11]. By incorporating rigid body point tolerances in the rigid body displacement matrices and calculating mechanism solution loci for the prescribed rigid body positions under the tolerance limits, circle and center point regions can be calculated. R–R link solutions can then be selected from these regions. This paper considers two-phase fixed and moving pivot adjustment problems with fixed and adjustable crank and follower lengths.

Kinematic Synthesis of Adjustable RSSR-SS Mechanisms for Multi-Phase Finite and Multiply Separated Positions

This paper presents a new technique for synthesizing RSSR-SS mechanisms to achieve phases of rigid body positions, velocities and accelerations using the same hardware. This work considers two-phase fixed pivot adjustment problems with fixed crank and follower lengths. By specifying the R-S link joint axes, the constant length condition becomes the only design constraint for these links. The prescribed finite and multiply separated positions are then incorporated in the link constraint equations with respect to the prescribed coordinate frame for each link and the RSSR-SS mechanism joint variables and driving link parameters are calculated.

Instant screw axis point synthesis of the RRSS mechanism

Abstract This paper presents a precision point synthesis of the RRSS motion generator, by specifying a set of successive points to the instantaneous screw axis. The method involves synthesizing RRSS mechanisms to achieve prescribed crank and coupler displacement angles by incorporating instant screw axis (ISA) points in the fixed axode point polynomial and calculating the R–R and S–S link parameters of this mechanism. The synthesis is facilitated by specific geometry of the RRSS mechanism, where the fixed axode is calculated as intersection of the R–R member plane and the S–S member axis. The RRSS fixed axode point polynomial was developed using the Cosine law approach introduced by Muller [Kansas State University Special Report No. 21, June 1962]. Complete expansion of the developed RRSS fixed axode point polynomial reveals that it is of order 56.

Use of snap-fit fasteners in the multi-life-cycle design of products

This paper describes the design and the use of snap-fit fasteners for the multi-life-cycle design of products. The advantages, drawbacks and general design considerations on snap-fits are listed. The paper emphasizes that unlike the design and use of conventional fasteners, every snap-fit used in a product must be designed from scratch. Issues of material strength, fastener geometry, part stiffness, attachment strategy and manufacturability are addressed. This paper also presents the use of compliant mechanisms i.e., design for disassembly in designing a new single piece snap fit fastener, which might be injection molded as a one-piece flexible structure. The conceptual design of a new snap-fit fastener which can be disassembled by using a simple tool is presented.

Planar Four-Bar Motion Generation with Prescribed Static Torque and Rigid-Body Reaction Force#

Abstract In motion generation, the objective is to calculate the mechanism parameters required to achieve or approximate a set of prescribed rigid-body poses. This work introduces a new design constraint that considers driving link static torque for a given rigid-body load. By incorporating this new constraint into conventional planar four-bar motion generation models (Sandor and Erdman, 1984; Suh and Radcliffe 1978), planar four-bar mechanisms are synthesized to achieve not only prescribed rigid-body poses, but also to satisfy prescribed driver static torque for a given rigid-body load. The included example demonstrates the synthesis of a four-bar braking mechanism.

Kinematic Synthesis of Planar Four-Bar Mechanisms for Multi-phase Motion Generation with Tolerances

Abstract This article presents a new technique for synthesizing planar four-bar mechanisms to achieve phases of both precise rigid body positions and rigid body positions with tolerances. This method is an extension of the adjustable RRSS motion generation synthesis methods developed by the authors. (Russell, K., Sodhi R. S. (2001). Kinematic synthesis of adjustable RRSS mechanisms for multi-phase motion generation. Journal of Mechanism and Machine Theory 36:939–952; Russell K., Sodhi R. S. (2002). Kinematic synthesis of adjustable RRSS mechanisms for multi-phase motion generation with tolerances. Journal of Mechanism and Machine Theory 37:279–294.) By incorporating rigid body point tolerances in the rigid body displacement matrices and calculating mechanism fixed and moving pivot solution loci for the prescribed rigid body positions under the tolerance limits, circle and center point regions were calculated. The circle and center points for the crank and follower link solutions for the planar four-bar mechanism were selected from these regions. In this article, two-phase moving pivot adjustment problems with constant crank and follower lengths are considered.

On cam system design to replicate spatial four-bar mechanism coupler motion

The coupler motion of a planar four-bar mechanism can be reproduced by rotating the mechanisms moving centrode over its fixed centrode. For spatial four-bar mechanisms, coupler motion can be replicated by moving the mechanisms moving axode over its fixed axode in a screw motion – a combination of simultaneous rotations and translations. This study presents a model to calculate the fixed and moving axodes of the revolute–revolute–spherical–spherical (RRSS) mechanism – one of the most basic spatial four-bar mechanisms. The axodes are useful in producing the contact surfaces for a cam system to replicate RRSS coupler motion.