PRODUCT DESIGN

GLOSSARY OF SPRING TERMINOLOGY

active coils: those coils which are free to deflect under load.

baking: heating of electroplated springs to relieve hydrogen embrittlement.

buckling: bowing or lateral displacement of a compression spring; this effect is related to slenderness ratio L/D.

closed and ground ends: same as closed ends, except that the first and last coils are ground to provide a flat bearing surface.

closed ends: compression spring ends with coil pitch angle reduced so that they are square with the spring axis and touch the adjacent coils.

close-wound: wound so that adjacent coils are touching.

deflection: motion imparted to a spring by application or removal of an external load.

elastic limit: maximum stress to which a material may be subjected without permanent

set.

endurance limit: maximum stress, at a given stress ratio, at which material will operate in a given environment for a stated number of cycles without failure.

free angle: angular relationship between arms of a helical torsion spring which is not under load.

free length: overall length of a spring which is not under load.

gradient: see rate.

heat setting: a process to prerelax a spring in order to improve stress-relaxation resistance in service.

helical springs: springs made of bar stock or wire coiled into a helical form; this category includes compression, extension, and torsion springs.

hooks: open loops or ends of extension springs.

hysteresis: mechanical energy loss occurring during loading and unloading of a spring within the elastic range. It is illustrated by the area between load-deflection curves.

initial tension: a force that tends to keep coils of a close-wound extension spring closed and which must be overcome before the coils start to open.

loops: formed ends with minimal gaps at the ends of extension springs.

mean diameter: in a helical spring, the outside diameter minus one wire diameter.

modulus in shear or torsion (modulus of rigidity G): coefficient of stiffness used for compression and extension springs.

modulus in tension or bending (Young’s modulus E): coefficient of stiffness used for torsion or flat springs.

moment: a product of the distance from the spring axis to the point of load application and the force component normal to the distance line.

natural frequency: lowest inherent rate of free vibration of a spring vibrating between its own ends.

pitch: distance from center to center of wire in adjacent coils in an open-wound spring.

plain ends: end coils of a helical spring having a constant pitch and with the ends not squared.

plain ends, ground: same as plain ends, except that wire ends are ground square with the axis.

rate: spring gradient, or change in load per unit of deflection.

residual stress: stress mechanically induced by such means as set removal, shot

peening, cold working, or forming; it may be beneficial or not, depending on the spring application.

set: permanent change of length, height, or position after a spring is stressed beyond material’s elastic limit.

set point: stress at which some arbitrarily chosen amount of set (usually 2 percent)

occurs; set percentage is the set divided by the deflection which produced it.

set removal: an operation which causes a permanent loss of length or height because of spring deflection.

solid height: length of a compression spring when deflected under load sufficient to bring all adjacent coils into contact.

spiral springs: springs formed from flat strip or wire wound in the form of a spiral,

loaded by torque about an axis normal to the plane of the spiral.

spring index: ratio of mean diameter to wire diameter.

squared and ground ends: see closed and ground ends.

squared ends: see closed ends.

squareness: angular deviation between the axis of a compression spring in a free

state and a line normal to the end planes.

stress range: difference in operating stresses at minimum and maximum loads.

stress ratio: minimum stress divided by maximum stress.

stress relief: a low-temperature heat treatment given springs to relieve residual

stresses produced by prior cold forming.

torque: see moment.

total number of coils: the sum of the number of active and inactive coils in a spring

body.

Feed mechanism

The movement of the tool relative to the work is termed as “feed”. A lathe tool may have three types of feed-longitudinal, cross, and angular. When the tool moves parallel to the lathe axis, the movement is termed as longitudinal feed and is effected by the movement of carriage. When the tool moves at right angles to the lathe axis with the help of the cross slide the movement is termed as cross feed, while the movement of the tool by compound slide when it is swiveled at an angle to the lathe axis is termed as angular feed. Cross and longitudinal feed is both hand and power operated, but angular feed is only hand operated.

The feed mechanism has different units through which motion is transmitted from the head stock spindle to the carriage. Following are the units:

1.End of bed gearing

2.Feed gearbox

3.Feed rod and lead screw

4.Apron mechanism

End of bed bearing

The gearing serves the purpose of transmitting the drive to the lead screw and feed shaft, either direct or through a gearbox. In modern lathes, tumbler gear mechanism or bevel gear feed reversing mechanism is incorporated to reverse the direction of feed.

Tumbler gear mechanism

Tumbler gear mechanism is used to give the desired direction of movement to the lathe carriage, via lead screw or the feed shaft. The tumbler gearing comprise of two pinions mounted on a bracket. The bracket is pivoted about the 1st stud shaft. The design provides three positions of bracket: forward, neutral, and reverse. With the forward position, only one gear train, and the lathe carriage is moved towards the headstock. With the introduced only to reverse the direction of rotation, and the carriage is neutral position, the spindle is disengaged from the lead screw is disengaged from the lead screw or feed shaft gearbox.

Bevel gear feed reversing mechanism

The tumbler gear mechanism being a non-rigid construction cannot be used in a modern heavy-duty lathe. The clutch-operated bevel gear feed reversing mechanism incorporated below the headstock or in apron provides sufficient rigidity in construction.

Specific design example

For a specific design example, we have extracted the design of the general size and shape of the standup panels for the control room. This design problem involved many designers, and several design changes occurred during the period over which we observed the project. For this reason, this example is rich enough to allow us to discuss the "ne details of design process. Fig. 1 shows the development of this design along a four-month timeline.

Standup panels are the large panels typically located along the walls of a control room on which meters and controls and alarms are placed. When the "eld study began, an initial design of the panel pro"le already existed.

For ergonomists, one of the main concerns is that controls and meters are readable and reachable. Therefore,

this "rst design was based on anthropometric data provided in the standard IEC-964. Further work with this data set re"ned the design of the panel. The analysis was then documented.

The data used from the international standard, however, was drawn from an American population. Two

months later, speci"c anthropometric data for the customer 's population were received from the customer.

This population's dimensions were somewhat smaller than the IEC-964 data. Accommodating this new data set would have required redesigning the control panels to be shorter by about 1 in (2.54 cm). The changes were considered to be insigni"cant and were noted but not made.

Two weeks later, it was discovered that the current panels would not "t through the hallways of the building,

as designed. For shipping purposes, the panels had to be resegmented. The panels were redesigned so that they could be segmented and shipped through the hallways of the building.

Near the end of August, a design document was issued to the customer illustrating the panel designs. The customer felt that the panels `looked too smalla and, upon learning that the panels were designed to meet anthropometric criteria, established a minimum height requirement for their operators, thereby cutting o! the lower end of the anthropometric data set. The panels were then redesigned to be taller.

The "nal change observed during the "eld study occurred due to a manufacturing consideration. It was decided that the panels would be constructed from mosaic material a modular construction of small blocks covered with plastic that would give #exibility in layout as it would permit modi"cations to be made. The material, although it could be cut, came in "xed sizes, one of which was just slightly larger than the size of the board. To make the manufacture of the panels easier and cheaper, the board was extended once again (Fig. 1).

As illustrated in the example, many practical changes happen during the course of a design. Although ergonomists may strive for the optimal ergonomic design, there are constraints that prevent the locally optimal

ergonomic design from being a globally optimal solution for the design problem.

The carriage of a lathe

The carriage of a lathe has several parts that serve to support, move and control the cutting tool. It consists of the following parts: 1.Saddle 2.Cross-slide 3.Compound slide or compound rest 4.Tool post and 5.Apron Saddle The saddle is an H-shaped casting that fits over the bed and slides along the ways. It carries the cross slide and tool post. Some means are generally provided for locking the saddle to prevent any movement when surfacing operations are carried out. The cross slide The cross-slide comprises a casting machined on the under aside for attachment to the saddle and carries locations on the upper face the tool post or compound rest. The crosspiece of the saddle is mechanized with a dovetail way, at right angles to the center axis of the lathe, which serves to guide the cross-slide itself. The compound rest The compound rest or compound slide is a mounted on the top of the cross-slide and has a circular base graduated in degrees. It is used for obtaining angular cuts and short taper as well as convenient positioning of the tool to work. By loosening two setscrews, which fit in a v- grove around the compound-rest base, the rest slide may be swiveled to any angle within circle. There is no power feed to the compound rest and it is hand operated. The compound rest handle is also equipped with a micrometer dial to assist in determining the depth of the cut. After necessary setting the compound slide is locked solid with its base. The tool post This is located on the top of the compound rest to hold the tool enable it to be adjusted to a convenient working position. Following are the common tool post: 1.Single screw tool post 2.Four bolt tool post 3.Open side tool post 4.Four way tool post The apron The apron is fastened to the saddle and hangs over the front of the bed. It contains gears, clutches, and levers for operating the carriage by hand and power feeds. The e apron also contains function clutches for automatic feeds. In addition, there is a split nut which engages, when required with the lead screw, when cutting either internal or external threads. The lay out of the apron includes an inter locking device which prevents the simultaneous engagement of the feed shaft and the lead screw. The apron handle wheel can turned to move the carriage back and forth longitudinally by hand. The complementary motion to this is obtained by the cross- feed handle, which moves the cross- slide back and forth across the saddle. The handle wheel is connected via a pinion meshing with a rack fitted to the lathe bed Usually a chasing dial or thread cutting dial is fitted either to the side or top of the apron and consists of a graduated dial. It has entirely independent drive provided by a worm wheel, which is a constant mesh with the lead screw.

fr. NTTF ( NETTUR TECHNICAL TRAINING FOUNDATION)

PRODUCT DESIGN

SAMPLE DESIGN

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GLOSSARY OF SPRING TERMINOLOGY

Feed mechanism

Specific design example

The carriage of a lathe

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