Print element with plural type layers of varying thickness

A print element for a printer having a multiplicity of type members arranged in a plurality of layers are supported on a support member or support members and one of said type members is brought to a predetermined printing position in which the type member strikes a sheet of paper wound on a paper to carry out printing by the type member. The thicknesses of the type members are varied from one another depending on the position in which each type member is arranged and on the vertical dimension of each type member. And when the type members are disposed in two layers on the forward ends of spokes extending from a center hub of the print element, and the back of the support member is struck by a print hammer to carry out printing by the type member, the support member has its thickness varied in accordance with the thickness of the type member supported thereon, so that the sum of the thickness of each type member and the thickness of the support member is substantially constant for all the type members.

BACKGROUND OF THE INVENTION 
This invention relates to a print element for use with a printer having a 
multiplicity of type members arranged in a plurality of layers. 
Generally, in a printer such as a serial impact printer, a print element is 
used. The print element may be of different types. FIG. 1 shows a print 
element in the form of a disc type print wheel generally referred to as a 
daisy wheel including a plurality of spokes 2 extending radially from a 
center hub and supporting at their forward end portions type members 5 and 
6 located on support members 3 and 4 respectively and arranged in a 
plurality of circumferences. FIG. 2 shows a bowl type print wheel 
including a plurality of spokes 2 arranged around a cylindrical or 
frustoconical surface and supporting at their forward end portions a 
multiplicity of type members 5 and 6 supported on the support members 3 
and 4 respectively. Said daisy wheel and bowl type print wheel are 
generally referred to as petal type print wheels. In many of these type 
wheels, type members are arranged in two layers on one spoke in order to 
contain many characters necessary for printing. FIGS. 1 and 2 show 
examples of this arrangement of the type members. When the type members 
are disposed in a plurality of layers, the print wheel is mounted through 
a universal joint on the drive shaft of a selection motor in the case of 
the disc type print wheel shown in FIG. 1. In the case of the bowl type 
print wheel shown in FIG. 2, the wheel is mounted slidably on the drive 
shaft and the print wheel is shifted in the vertical direction by a print 
signal, to bring one of the type members 5 and 6 to a predetermined print 
position as shown in FIG. 3 and give an impact to the back of the support 
members 3 or 4 by a print hammer 10. Thus the type member 5 or 6 strikes 
through an inked ribbon 9 a sheet of paper 8 wound on a platen 7 to carry 
out printing. 
In this case, the adjacent type members are preferably located as close as 
possible to each other to reduce the radius of the print wheel, in order 
to make it possible for the print wheel to rotate at high speed by 
reducing the moment of inertia of the print wheel as a whole. In the print 
wheel of the type in which the type members are arranged in two layers in 
each of the spokes, it is necessary that the spacing interval between the 
print members of the two layers be minimized so as to reduce the time 
required for the shifting of the print wheel in a vertical direction and 
simplify the shifting mechanism by reducing the distance covered by the 
vertical shifting movement of the type wheel. 
The type members 4 and 5 have their surfaces concavely curved as shown in 
FIG. 4 to conform to the shape of the outer peripheral surface of a platen 
7. Generally, the height a of the central portion of each of the type 
members 5 and 6 from the surface of one of the support members 3 and 4 has 
had the same value for all the type members 5 and 6. Thus in the case of a 
print wheel having the two type members 5 and 6 on each of the spokes 2, 
the central portions of the two type members 5 and 6 naturally have the 
same value a. When this is the case, there would be the possibilities that 
as printing is carried out on the paper 8 by the type member 6 as shown in 
FIG. 5, the spoke 2 would be flexed toward the platen 7 in the forward end 
portion thereof as seen from the type member 6 by inertia at the time an 
impact is given to the type member 6 and a lower end portion 5a of the 
type member 5 would also strike the paper 8 if the distance between the 
two type members 5 and 6 were reduced. This would result in the spoiling 
of the paper 8 by ghost printing. 
The position in which a type member is struck by the print hammer 10 is 
constant at all times because a hammer guide 11 (see FIG. 3) is fixed, and 
impact is given by the print hammer 10 to the center of the back of each 
of the support member 3 and 4. The type members 5 and 6 on the surfaces of 
the support members 3 and 4 respectively differ from each other in 
position or size with respect to the center at which the impact is given 
to the back of each support member, depending on whether the type members 
are characters or symbols. Assume that, as shown in FIG. 6, a characters, 
such as 1/4, which is more elongated downwardly than ordinary capital 
letters, such as A, B and C, is supported on the support member 3 as the 
type member 5, and the character 6 supported on the support member 4 is a 
type member, such as ",", which is disposed in a position below the center 
of the support member 4 at which impact is given. In the case of a spoke 
which supports type members of this type of combination, no problems are 
raised when printing is carried out by the type member 5 as shown in FIG. 
7 because the distance l.sub.1 between an impact center .phi..sub.1 and an 
upper end 6b of the type member 6 and hence the gap .delta..sub.1 between 
the upper end 6b of the type member 6 and the platen 7 is large. However, 
when printing is carried out by the type member 6 as shown in FIG. 8, the 
distance l.sub.2 between an impact center .phi..sub.2 and a lower end 5a 
of the type member 5 is small and hence the gap .delta..sub.2 between the 
lower end 5a of the type member 5 and the platen 7 is small. Thus even if 
the support members 3 and 4 of the two type members 5 and 6 are formed 
integrally with each other, ghost printing tends to be performed by the 
lower end portion 5b of the type member 5. 
The characters and symbols that extend downwardly more than the ordinary 
capital letters include, besides the aforesaid "1/4", "2/4", "1/2", "3/4", 
"/", "(",")", "{","}", ".delta.", " ", "#", etc. The symbols whose type 
faces deviate upwardly or downwardly from the impact center include, 
besides the aforesaid ",", ".", "'", " / ", " ", """, ".about.", " ", etc. 
SUMMARY OF THE INVENTION 
Accordingly this invention has as its object the provision of a print 
element of simple construction capable of eliminating the trouble of 
producing ghost printing when the type members are arranged in two layers 
in the print wheel. 
The aforesaid object is accomplished according to the invention by varying 
the thicknesses of the type members in accordance with the position in 
which the type members are located, deviation of the type members from the 
center in which an impact is given by the print hammer, or the vertical 
dimensions of the type members. By altering the thicknesses of the backs 
of the support members for the type members so as to render substantially 
uniform the total of the thickness of each type member and the thickness 
of the support member for all the type members, it is possible to obtain a 
uniform printing pressure and a uniform print density. Uniform printing 
pressure can be obtained by controlling the force of impact for 
accelerating the print hammer in accordance with the thicknesses of the 
type members.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the invention will now be described by referring 
to the drawings. FIGS. 9 and 10 show a type section on a spoke of a petal 
type print wheel comprising one embodiment of the invention. A spoke 14 
has formed at its forward end portion an outer support member 15 and an 
inner support member 16 supporting thereon a type member 17 representing a 
character "A" and a type member 18 representing a character "B" 
respectively. As shown in FIG. 10, the type member 17 has a thickness b 
and the type member 18 has a thickness c which is greater than b. The 
difference between b and c is set at a value such that when printing is 
carried out by the type member 17 as shown in FIG. 11, an upper end 
portion 18b and a platen 11 define therebetween a gap .delta..sub.3 of a 
suitable size. The back of the support member 15 is displaced rearwardly 
as compared with the back of the support member 16 a distanced 
substantially equal to the difference between b and c. 
Thus when printing is carried out by the type member 17 as shown in FIG. 
11, the upper end portion 18b of the type member 18 becomes close to the 
platen 7 but a portion of the spoke 14 on the type member 18 side is not 
flexed when an impact is given to the type member 17 so that the spoke 14 
is prevented from flexing toward the platen 7. Thus the gap .delta..sub.3 
of a suitable size is defined between the upper end portion 18b of the 
type member 18 and the platen 7 and no ghost printing takes place. 
Meanwhile when printing is carried out by the type member 18, the type 
member 17 located on the free end side of the spoke 14 tends to flex 
toward the platen 7 due to inertia at the time an impact is given to the 
type member 18. However, since the type members 17 and 18 have thickness b 
and c respectively which are in the relation b&lt;c, so that a lower end 
portion 17a of the type member 17 which is closest to the type member 18 
to be used for printing and the platen 7 define therebetween a gap 
.delta..sub.4 which is sufficiently large to prevent ghost printing to be 
caused to occur by the lower end portion 17a of the type member as shown 
in FIG. 12. 
Assume that as shown in FIG. 13 the support member 15 located on the outer 
side supports thereon a type member 19 representing a character "1/4" 
which has a larger length than ordinary capital letters and the support 
member 16 located on the inner side supports thereon a type member 20 
representing a character "E" of ordinary length. In this case, the 
distance l.sub.2 between the impact center .phi..sub.1 of the type member 
20 and a lower end portion 19a of the type member 19 is smaller as shown 
in FIG. 16 than would be the case if the type members represented 
characters, such as "A" and "B", which have an ordinary length. Thus when 
printing is carried out by the type member 20, ghost printing would tend 
to take place as the lower end portion 19a of the type member 19 strikes 
the platen 7, if the type member 19 and 20 were of the same thickness. By 
increasing the thickness c of the type member 20 a suitable amount as 
compared with the thickness b of the type member 19 as shown in FIG. 16, a 
gap .delta..sub.5 of a suitable size can be defined between the lower end 
portion 19a of the type member 19 and the platen 7, thereby avoiding the 
occurrence of ghost printing by the lower end portion of the type member 
19. There will be no need to describe that the thicknesses b and c of the 
type members 19 and 20 are selected such that when printing is carried out 
by the type member 19 as shown in FIG. 15, the gap .delta..sub.3 of a 
suitable size is defined between an upper end portion 20b of the type 
member 20 and the platen 7. 
When the outside support member 15 supports a type member 19 representing a 
character of a large size, such as "1/4", the inner support member 16 
advantageously supports, as shown in FIGS. 17 and 18, a type member 21 
representing a character such as "," which is disposed below the impact 
center .phi..sub.1. This enables a gap .delta..sub.6 defined between an 
upper end portion 21b of the type member 21 and the platen 7 to be 
increased as shown in FIG. 19 when printing is carried out by the type 
member 19. Conversely, when the inside support member 16 supports a type 
member 22 representing a character of a large length, such as "/", as 
shown in FIGS. 20 and 21, the outside support member 15 advantageously 
supports a type member 23 representing a character, such as " " which is 
located higher than the impact center .phi..sub.1. In this case, one has 
only has to reduce the thickness b of the type member 22 below the 
thickness c of the type member 23 as shown in FIG. 21. 
In the embodiment shown and described hereinabove, a gap of a suitable size 
can be provided between either of the upper and lower type members and the 
platen 7 without regard to the type member used for carrying out printing 
by varying the thicknesses of the two type members 15 and 16 supported on 
one spoke, depending on the position in which the type members are 
located, the degree of deviation of the type members from the center of an 
impact given by the print hammer or the vertical height of the type 
members, or whether the type members are generally of a large size, such 
as "A" and "B" or the type members have a large vertical dimension, such 
as "1/4", "/", "[", "]", etc. This is conducive to prevention of the 
occurrence of ghost printing. 
FIG. 3 shows a known construction of a print hammer device of a printer. A 
hammer 10 is connected to the forward end of a plunger 12a of a solenoid 
12 as a unit and normally located in an inoperative position away from the 
platen 7 by the biasing force of a compression coil spring 13. Upon the 
solenoid 12 being energized for a short period of time by a printing 
signal, the plunger 12a having the hammer 10 at its forward end is 
actuated and moves toward the platen 7 against the biasing force of the 
coil spring 13. Even after the solenoid 12 is de-energized, the hammer 10 
and plunger 12a move forwardly by inertia. However, their speed of forward 
movement is gradually reduced by the biasing force of the coil spring 13 
until the hammer 10 strikes either one of the backs of the support members 
3 and 4 with a suitable force, so that printing is carried out on the 
paper 8 through the inked ribbon 9 as one of the type members 5 and 6 is 
brought into contact with the platen 7 through the paper 8 and the inked 
ribbon 9. 
Thus if the support members have a constant thickness when the outside type 
member and the inside type member have different thicknesses according to 
the invention, the hammer would have to travel a greater distance when it 
strikes the back of the type member of a smaller thickness than when it 
strikes the back of the type member of a smaller thickness than when it 
strikes the back of the type member of an ordinary thickness before the 
type member is brought into contact with the paper on the platen. The 
result of this would be that due to a reduction in the impact with which 
the hammer strikes the back of the support member, a variation might be 
caused to occur in the density of the printed characters depending on the 
thicknesses of the type members. 
According to the invention, when the type members have a thickness b which 
is smaller than the ordinary thickness c, the thickness of the support 
member 15 or 16 is increased by an amount substantially equal to the 
difference in the thickness of the type members or c-b in all the 
embodiments of the invention. Thus in a print wheel according to the 
invention the sum of the thickness of each type member and the thickness 
of its support member is constant for all the type members supported on 
the print wheel. By this arrangement, the stroke of the hammer can be made 
constant from its inoperative position to the position in which all the 
type members strike the surface of the paper on the platen, and 
consequently the force of impact with which the hammer strikes the back of 
each support member can be made equal. This is conducive to elimination of 
differences in the density of the printed characters even if the 
thicknesses of the type members are varied. 
It is also possible to eliminate the occurrences of different density in 
the printed characters by effecting control electrically without varying 
the thicknesses of the support members. More specifically, since the 
address of the type members is fixed for the print element or print wheel, 
it is possible, by providing a control section 25 and a memory 26 as shown 
in FIG. 22, for the control section 25 to receive the type member address 
and printing information therefor from the memory 26 upon inputting of an 
encoded print instruction. It is, therefore, possible to control the 
printing pressure applied by the hammer 10 by controlling the period of 
time during which a current is passed to the solenoid (12 shown in FIG. 
3), in addition to control the direction and amount of rotation of the 
selection motor. 
In the description of the embodiment shown in the accompanying drawings, a 
petal type print wheel has been used as a print element in which the 
present invention is incorporated. However, it is to be understood that 
the invention is not limited to the specific form of the print wheel, and 
that the present invention can have application in a cylinder type print 
head 29 shown in FIG. 23 in which type members 27 and 28 are arranged in 
two layers or upper and lower layers and a glass-type print head 32 shown 
in FIG. 24 in which print members 30 and 31 are arranged in two layers or 
upper and lower layers, depending on the positions and sizes of the type 
members 27 and 28 and 30 and 31.