Feld-Hell design evolutions

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The Feld-Hell was manufactured from 1935 through 1944 (though based on my database of about 100 machines, 1937 appears to have been skipped). Clearly, as the war progressed, materials and skilled labor became increasing scarce. Hence, the design and manufacturing was rationalized, starting in 1943. The resulting design variants are discussed below. In particular, this affects the:

Some Feld-Hell machines have a printer module with an external thumbwheel to adjust height of the printer hammer. This is not a Feld-Hell variant, but a replacement printer module taken from a 1930s/40s T empf 14 "Presse Hell" machine. It has the same "form-fit-function". That is: it is fully compatible, with the same dimensions, mechanical and electrical interfaces, and solenoid.

T empf 14 "Presse Hell" printer

T empf 14 printer module installed on a Feld-Hell machine

There are also derivative models, rather than design variants of the standard Feld-Hell:

There is a round 12-pin connector ("Zwölffachsteckdose", "Steckerbuchse", "Anschlußbuchse") located at the lower right-hand corner of the Feld-Hell's amplifier box. This connector provides access to the contacts of the character-drum, for keying a CW transmitter. There are also outputs for accessories such as a remote control unit and a keying device for high-power CW transmitters: constant 900 Hz tone, 12 Vdc and 165 Vdc.

This connector has a brown, dark brown, or black bakelite insert. The natural color of the bakelite resin is brown. Black is obtained with a molding powder, hence more expensive. I have seen at least one Feld-Hell machine where the connector had a white insert. This may have been a very early model, with a ceramic insert (note that the first RV12P4000 tubes (1935) had a white porcelain base).

It appears that the connector (and associated internal wiring) was eliminated some time during 1944. The majority of field operations was probably over phone lines, in which case the connector is not used. Eliminating the connector saves material and labor. Once the connector was eliminated, is was also no longer necessary to machine a hole for it in the front of the die-cast box, and install a cover plate. Further savings!

In 1938, the shape of the knob of the main switch was changed, from asymmetrical to symmetrical. Possibly the latter is more rugged, as force is not applied off-center. At the same time, the knob material was changed from cast metal to bakelite - possibly a cost reduction. Note that the actual switch is not a rotary switch at all: its is a simple slide switch that is actuated by an eccentric lever on the shaft of the knob!

Knob of the main switch: before 1938 (left) and from 1938 onward (right)

There is a red indicator lamp, just above the main switch. It is illuminated when the main switch is in the upright "Bereit" position ("Standby").

A number of units from the final production year (1944) do not have the lamp installed, nor is there a hole in the front face of the box.

At best, the lamp is a "nice to have" feature, as it does not illuminate when there is no circuit-continuity through the motor's carbon brushes and commutator. Other than that, it provides no useful information - the voltmeter already indicates the 12 volt power level. Hence, in late-war Feld-Hells, the lamp was no longer installed (and wired) and no hole was machined into the front of the amplifier box. Material and labor savings!

The connector block of the amplifier box has jacks for connecting the loudspeaker/headphones output of a radio receiver ("Empfänger"). Some units built during the final production years (1943/44) have a small bakelite plate that covers the jacks. I do not know whether the jacks (and wiring) are not installed, or only covered up. For operation over phone lines ("Drahtbetrieb"), this interface is not required. The Feld-Hells in which this modification was made, typically also do not have the 12-pin round connector for interfacing to a CW-transmitter.

Connector block with "Empfänger" jacks (1938) and with covered jacks (1943)

The connector block is made of light brown, dark brown or (more expensive) black bakelite. In my database, there appears to be no consistent switch-over between colors. Brown and black connector blocks are found in pre-war machines as well as late-war machines. There also appears to be no correlation between the bakelite color, and the manufacturer of the amplifier box (Siemens-Halske vs. Mende Radio).

The Feld-Hells have three binding-post terminals: one for case-ground (earth), and a dual binding-post for the La - Lb/E phone line wires.

In the 1936 model, the binding posts are all-metal. The dual binding-post is on the same rectangular bakelite block as the phone line jack, the radio and headset jacks, and the 12 volt power connector. The case-ground binding-post can only accommodate a wire, whereas the dual binding-post can also be used with banana plugs.

The binding-posts of the 1938-1944 models have a bakelite knob for (un-)tightening. The dual binding-post is mounted on a separate bakelite block. The terminal for case-ground is also mounted on a bakelite block, and can also accommodate a banana plug.

1936

All-metal binding posts

1938-1944

Bakelite binding posts

There is a toggle switch on top of the electronics box. It is labeled "Sammler – Netzgleichrichter" that is "battery – transformer-rectifier unit". In the "Netzgleichrichter" position, a 5 Ω resistor is placed in series with the 12 Vdc power input when the main switch is in the "Bereit" position. This is to support an older model Feld-Hell-specific external power supply, that had poor voltage regulation under low-load conditions.

According to the 1941 Feld-Hell manual (line item 44), "newer" Feld-Hell versions do not have this switch. So far, I have found only one amplifier box without the toggle switch. See photo below. There are no markings on the paint, nor loose wires, that suggest that the switch was removed after installation. The amplifier box in question was manufactured in 1943.

The Feld-Hell has a compartment for the paper tape rolls. It is located below the keyboard. The compartment has the following parts:

When the upper paper cassette of the Feld-Hell runs out of paper, the lower cassette must be opened, the paper tape pulled through a slit on the left of the cassette, and the paper passed underneath the printer spindle. Exactly the same procedure is followed when simply putting a new roll of paper in the upper cassette. So, in fact, the second (= lower) paper cassette of the Feld-Hell is just a convenient place to store a spare roll of paper tape. Pure luxury!

Towards the end of the war, this luxury became unaffordable. Some of the machines built in 1943/44 dispensed with the compartment cover plate, the lower paper tape cassette, the mounting hardware and release button for the lower cassette, and the cover plate underneath the compartment.

1943 model - not installed: hinged cover, bottom plate, lower paper cassette, its tracks and release button

In a final rationalization step, the release button for the (no longer installed) compartment cover plate was no longer installed. Also, drilling and machining steps for for the paper exit slit and the (no longer installed) cassette release button were skipped.

1944 model - as 1943 model above, minus cover release button in the base of the keyboard, the hole for the cassette release button, and paper exit slit

Gear train of the Hell Feldfernschreiber

All worms are made of steel, and are single-start. All worm gears are made of "Hartgewebe" (HGW): cotton-reinforced phenol-formaldehyde resin. All worm gear wheels are helical gears: the teeth are not parallel to the gear's axis of rotation, but set at an angle. The teeth are actually very slightly curved, and are in fact a small segment of a helix. Helical gears run more smoothly and quieter than simple "straight cut" spur gears. However, they also exhibit more sliding friction between meshing teeth than spur gears - which is why they are lubricated.

At some point in time, the geometry of the worms and corresponding worm gears were changed. Their pitch (the on-center distance between adjacent teeth of the gears, and the coarseness of the thread of the worms) was increased. A the same time, the diameter of the worms was increased and that of the worm gears decreased decreased accordingly - the geometry and position of the drive shafts remained unchanged.

The gear ratio of a worm gear set is obtained by dividing the number of teeth of the worm gear-wheel, by the number of starts (threads) of the worm. In our case, all worms are 1-start. So the gear ratio is simply the number of teeth of the worm gear-wheel. Depending on the gear wheel, the number of teeth was reduced by 10-30%. I am still trying to figure out the technical reasons for these changes, and why it did not affect the rpm of the printer spindle and the character drum... These gear changes were not made for economic reasons.

Worms on the main drive shaft (coupled to motor) - 1938 (bottom) vs. 1944 (top)

Worm-gear on the drive shaft of the character drum - 1938 (above) vs. 1944 (below)

Worm-gear on the drive shaft of the printer spindle - 1938 (above) vs. 1944 (below)

One more change was made to the worm gears of the drive shaft of the character drum and of the printer spindle. See photos above. Originally, these gears were affixed to the drive shaft with a tapered pin through the hub of the gear wheel and through the shaft. Making a tapered hole requires a special tool: a tapered reamer. Also, the taper of the hole through the shaft must match the taper through the hub of the gear wheel. This usually means that the worm gear must already be on the shaft when the hole is reamed. All this takes time during production, and a skilled machine operator. Removing the taper pin - without damaging the gear or the shaft - is also not easy. The entire process can be greatly simplified by mounting these gearwheels with a standard, cheap, mass-produced M3 screw. Yes, this is technically somewhat inferior. But now the holes through the shaft and the gearwheel can be made separately (time and location), with a simple drill and a tap, and without specialized labor. Note that the worm gear on the paper transport drive shaft is screw-mounted in all Feld-Hells. This has to do with the fact that the gear is installed after the shaft is installed in the gear box.

Most bearings are of the standard "single-row deep-groove" type. However, in some Feld-Hell machines, one or more self-aligning bearings ("Pendellager") may be found. These bearings have two rows of balls. They share a concave, spherical raceway in the outer ring of the bearing. During normal operation, these bearings can accommodate a couple of degrees of angular misalignment between the outer ring of the bearing, and the shaft. Such misalignment occurs when a shaft has a bearing at both ends, and the bores into which the bearings are installed are not concentric ( = coaxially aligned). In this case, a self-aligning bearing should actually be used at both shaft ends.

Single-row deep-groove ball bearing

Double-row self-aligning ball bearing

(race turned 90 degrees)

As the photo above clearly shows, self-aligning bearings can handle much more misalignment during installation. This is quite convenient when putting the multi-shaft gear box of the Feld-hell back together!

Self-aligning bearings have a friction coefficient that is about 20% lower than that of a single-row deep-groove bearing. This is important on the shaft with the highest speed: the input shaft (3600 rpm). Especially given the fairly low power of the Feld-Hell motor.

The character drum drive shaft has a large steel gear wheel on the outside of the gear box. It meshes with a similarly large gear wheel on the shaft of the character drum. There bearing behind this gear wheel can be adjusted up and down, to adjust the meshing. This means that both bearings on the shaft of the character drum are forced to be misaligned in their socket, or misalignment force is applied to the inner ring of the bearing. This is why some Feld-Hells have one or two self-aligning bearings on the character drum. On this shaft, the original bearings in my 1938 Feld-Hell are not self-aligning, and their fit into the bores is rather loose.

Sometimes the drive shaft of the printer spindle also has self-aligning bearings. But not in my machine. Given the trouble that I had removing and reinstalling this shaft, it is definitely a good candidate.

Self-aligning bearings can only support about 70% of the radial load of a regular bearing, and only about 30% of the axial load. And they are obviously more expensive.

	round connector with a pivotable cover,
	round connector with wire-retainer for the mating male connector,
	no round connector - a square plate covers the hole in the front face of the amplifier box,
	no round connector, and no hole in the front face of the box.

	a hinged cover plate to hide the cassettes,
	a release button for this cover plate,
	two paper tape cassettes,
	sliding tracks for each cassette,
	a release button for each cassette,
	a slit to the left of each cassette, where the paper tape leaves the cassette compartment,
	a cover plate underneath the cassette compartment.

	input shaft. It is installed vertically, and coupled to the motor. This shaft has two worms.
	character-drum drive shaft. It has one worm gear (a.k.a. worm wheel) that meshes with the larger (upper) worm of the input drive shaft. One end of the shaft has a large metal gear, that meshes with a pinion gear on the character drum. A worm at the opposite end of the shaft drives the paper transport shaft. Note that the term "worm gear" is often incorrectly used to denote the combination of a worm and a worm wheel.
	paper transport drive shaft. It has a worm gear that meshes with the worm at one end of the character-drum drive shaft.
	printer spindle drive shaft. It has one worm gear that meshes with the smaller (lower) worm of the input drive shaft. The crown gear at the opposite end of spindle drive shaft meshes with the pinion of the actual printer spindle.

	motor-generator: 2
	input drive shaft: 2
	spindle drive shaft: 2
	character drum drive shaft: 2
	character drum shaft: 2
	character drum cam-followers: 2
	ink roller holder: 1

	TBS24b, built by the Hell company itself, rather than Siemens-Halske
	Hungarian post-war Feld-Hell