Effect of platen hardness on the sound of a typewriter

The sound that typewriters make is determined by the overall design - and by the rubber of the platen or cylinder. To be able to get an impression of the effect of platen rubber, one machine was tested with two new platens of  different hardness.

Especially machines from the 1930s and older will have a platen covered with natural rubber that can harden over time. This hardening makes the typewriter louder and 'sharper'. Additionally a rock-hard platen is harder on the type-slugs, possibly increasing the chance of damage and reduces print-quality. An older platen's surface will often be marked by the type-slugs impact, with pitting of the characters in the surface. 

Re-surfacing a rock-hard platen with new rubber is thus a good thing to do, if feasible. A hardness of between Shore 85 and 90 is I think generally advised for typewriters. When re-surfacing; what would however be the effect of choosing a softer or harder platen if wanting a less-loud machine?

To assess the impact on sound of only the platen hardness, a Remington Portable 2 typewriter was fitted with a new platen with a hardness of about Shore 90. The sound of typing a line halfway down a page was recorded. Then the platen was swapped-out with a new, very soft platen of Shore 80. Again the sound of typing a line halfway the page was recorded in the same set-up. Then the sound recordings were compared, using the Audacity program.

The typewriter was placed on a felt-mat to dampen resonance of the mechanism with the table, relative to the sound of the impact of the type. With its very exposed typebars and typing-point, the sound of the type striking the platen should be dominant - more so than for later, more enclosed typewriters.

Seeing the spectrogram views for both platens side-by-side immediately confirms what could be heard; that the sound is different. The soft platen's energy-peak is at a lower frequency. The hard platen sounds sharp, a crisp clack. The soft platen is a more muffled pop or thup sound with the overall rattle of the machine more noticeable. (Sound snippets in a zip file.)

When comparing graphs of the spectrum of the sound from both platens sized to the same scale, it can be seen that the soft platen is different and indeed also less loud. The volume of the key-strike sounds is lower. The peak of the type-striking sound with a hard platen is from 4 to 6 kHz, whereas the soft platen's peak is in the 2 to 3 kHz range. 

The volume and spectrum of the sound below about 1 kHz is essentially identical for both platens - this is the relatively low-frequency rattle and clanking of the mechanism; noise made by the carriage, keys and whole internal mechanism. (A proper analysis of the captures with e.g. Octave could probably extract much more information, but that'd be taking things a bit far perhaps - not that this is, or is it...)

The graphs incidentally make clear that further reducing the sound of the type-strike becomes less relevant already at this soft-platen's level. The general noise of the whole mechanism is already a major factor for the perceived sound overall.

The graphs do confirm that indeed the typewriter becomes less loud with a softer platen. Having said that, the print quality with a very soft platen does suffer. Also the printing becomes more sensitive to the operator's touch.

For this Portable, the Shore 80 platen is definitely too soft and the print-quality is noticeably lower than the printing with the Shore 90 platen. There is markedly more 'ribbon-fuzz' around the character. With the soft platen the machine needs a very light touch to avoid embossing the paper, so only a fairly light imprint is possible. These drawbacks will almost certainly vary per typewriter model. Not every typewriter and/or type-slug will have the same sensitivity to the platen-material properties. (E.g. on the Underwood 5, a very soft platen worked great - on this RP2 it is too soft really.)

With the very soft platen, the effect of the touch of the typist has a stronger impact on the imprint - to get an even, constant 'blackness' a very even touch is needed. With a harder platen this effect is less pronounced and within a wider range of 'touch' the imprint remains reasonably constant.

Using an extra backing sheet with the soft platen helps to bring the print-qualities closer together.

So overall; as with most engineering things, it is a compromise. The conclusion for now is that softer is indeed quieter, but at the expense of print-quality.

Carriage height adjustment for the Underwood 3-bank portable

This 3-bank portable typewriter needed to be adjusted. The lowercase did not print fully (bottom-heavy) and the two shifts also did not line up correctly. The machine probably had something heavy stacked onto it or been generally knocked about to 'drive' the carriage lower. (To be fair - any typewriter that looks like this one did, should be expected to need some tuning and adjusting.)

Fortunately the Underwood Standard Portable Typewriter (3 bank) is entirely adjustable - once you find the right screws and order of adjusting them.

The basic approach is to first adjust the carriage baseline height so that the lowercase prints fully. Then the two shifts are adjusted to match the baseline. Finally the lock-functions for the shifts are adjusted. This sounds reasonably simple, but it took a few tries and pensive peering into the mechanism to get it right. (A typewriter repairman could have made some remarks on the fumbling about...  ...but we got there - I think :-)

In the Underwood 3-bank, the baseline is determined by the carriage resting on the Caps-shift levers. This Caps-shift lever in its turn rests on the adjusting screw. The shifted positions are determined by adjusting-screws that stop the upward movement of the lever and carriage. With these three adjustable 'stop-positions' the typewriter can be aligned completely

If the machine is a later model with also a right shift lever, then adjusting-screw 1 in the picture below is probably the one to start with for the adjusting. If the machine has only the left shift-keys, probably start at screw 2 with the same procedure. Loosen the lock-nuts on all adjustment screws 1, 2 and 3 and loosen adjusting-screws 2 and 3, i.e. screw out a few turns to give space for the adjusting. The carriage height is now determined only by screw 1.

This screws in for raising the carriage, out for lowering the carriage. Judging the correct height by only a few characters is quite hard - even with the qpdb-set. When however a couple of lines of text are typed, it becomes easier to detect a bias of the type, e.g. a bottom-bias with the letters 'thicker' at the bottom or a top-bias with thicker/heavier ink at the top. In case of bottom-bias the carriage must be raised, with top-bias it must be lowered. 

Iterating slowly with ever smaller turns of the screw will get the paragraph typed without discernible bias in the printed text. Start e.g. with full turns up or down until overshooting to the other bias, then half a turn the other way, a quarter turn etc. When the paragraph types without bias, tighten the lock-nut to fix the adjustment in position. Best hold the screw with a screwdriver when tightening the lock-nut to prevent accidental turning of the screw and losing the adjustment.

Note: make sure that any bias is not caused by the ribbon. E.g. a ribbon-vibrator that is dirty or out of adjustment could cause the upper-end of characters to not print reliably and give the false impression of bottom-bias!

With the carriage now at its correct baseline height, adjust upwards the screw 2 for the left Caps-shift lever until the key has no 'empty travel' left. Note that there is an extra mechanism in this lever that also has some travel, so best keep an eye on the top of the adjusting-screw inside the machine as it touches the lever. Similarly the screw 3 can be adjusted upwards to remove 'empty travel' of the Fig-shift lever. Then these can then also be lock-nutted in position.

With the baseline now adjusted, the capitals have to be matched. For this, the font-panel of the typewriter should be removed. With the right tools it could probably be done with the panel in-place, but it is much better to work with the panel out of the way for better access and visibility of the parts. (Front panel removed by 5 screws, as in previous post.)

Note: if removing the front-panel, take care to make sure that at least one of the spool-ratchets is 'free', otherwise the ribbon-advance will pull on both spools, jam the ribbon-vibrator and mess up the whole adjusting of the carriage!

The Caps-shift position is determined by the adjustment-screws in a tilting bar on the left and right of the machine. First put the machine in Fig-shift, this tilts the bar forward and brings the Caps adjusting screws into view. The left screw is shown in the picture below, with a spanner of just the right size ready to loosen the lock-nut.

The upper- and lowercase can be tuned by adjusting both at the same time, or one by one. It seemed better to do the fine adjustment one at a time. The left screw is probably best adjusted with the carriage to the left (weight over the screw being adjusted). Typing pairs of Mm, Mn and Rr will quickly show if the lowercase needs to rise higher to match the uppercase or the other way around. With carriage to the right, the right-screw is given the fine-adjustment the same way. It is a bit of a roundabout way, needing to Fig-shift to get at the screws to make the adjustment, then back to baseline and uppercase. With again the approach of iterating with ever smaller turns of the screws, this Caps-shift adjustment should be fairly quick and straightforward.

The Fig-shift alignment is similar - its position is determined by two adjustment screws low in the machine just in front of the carriage. These can be easily reached with a screwdriver, however getting a spanner on the lock-nut is rather tricky. (Long needle-nose pliers may be needed; it is bad workshop-practice but may be the only way.) The picture below shows the left Fig-shift screw, the right screw is similarly inaccessible at the right side of the machine.

Using the comma (lowercase) with the semi-colon (Fig-shift) and the 2 with the m, the Fig-shift position is adjusted to match the baseline carriage height. This works the same way as the Caps-shift adjusting, by making sure the lowercase character rises just enough to match the position of the character printed in the shifted position.

With these seven adjustment screws now all tuned and the baseline, Caps-shift and Fig-shift at the correct (or good enough) setting, the lock-functions can be adjusted. These will likely no longer work as they should, because the retaining tab needs to match the shifted position of the key-lever. The Caps-shift lock bracket is held by by the screws indicated in the yellow oval, the Fig-shift lock by the screws in the blue oval (plus it may need the top of the Caps-shift screws loosened).

The little tab on the shift-lever needs to slide just under the bracket in the slot. The brackets can be adjusted up and down, having long slotted holes where they are screwed to the comb-plate.

With a little tweaking, these brackets can be fixed in a height that the tab slides nicely under the bracket and holds it in the shifted position without any 'sagging'. I.e. there should be no dropping of the carriage when it is held by the shift-lock. It should however be not so tight that a press on the key won't release the tab from the bracket. A bit of tweaking of their positions should make both shift locks work as intended again.

With the typewriter adjusted, it may settle a bit. Typing a page will show up any play or screws that weren't tightened as well as they should. Starting from an already (mostly) adjusted machine, further fine-tuning or fixing becomes easier. The end-result should be an Underwood 3-bank that will again type quite decently.