Idea behind twin tubs was simple
They preserved the water/soap/detergent savings of wringer washers but offered a safer means of extraction.
Quite honestly there was a hate brewing against mangles ever since the things arrived on scene; that chorus grew louder when electrically powered wringers came into existence. Simply put the things were dangerous, a necessity but never the less dangerous.
With a hand operated wringer at least one can stop turning quickly; OTOH many early powered wringers were simply not safe; they either had no easy way to stop and release the rollers and or the switches couldn't be easily reached before one was garroted, lost a limb or whatever.
Then there was the fact wringers aren't very good at extracting water. At best most compared to about 200rpm spin speed. Also because basically you are squeezing layers of fabric flat it causes all sorts of problems. Broken buttons, torn hems or other parts of textiles were common with both hand and electric powered mangles. Women would spend hours after laundry day mending tears and or replacing buttons damaged by wringing on laundry day. Some women/laundresses would remove all buttons from items before sending them to the wash, then sewed them back on afterwards.
Hard wringing also created deep creases that would have to be ironed out later.
Enter the spin drying tub! Just as with commercial laundries and their extractors it was now possible to remove water from laundry by centrifugal force. This resulted in less wear and virtually removed possibility of damaged buttons and other fastenings. Long as the thing was loaded properly.....
Maytag and others who stayed with wringer washers for decades kept at R&D to perfect the art. Balloon wringer rollers became standard. These were advertised as unable to break buttons/fastenings. Maytag went further by designing a mangle with one hard (black) roller and one softer. So instead of mashing laundry between two hard objects, the softer roller yielded. This supposedly removed risk of damage and all those hard creases.
Spin driers on twin tubs also allowed extraction of items that never would go through a wringer, or at least easily. Things like thick blankets, rugs, pillows, etc.... Instead of having to remove the feathers/down from a pillow (wringer washer), you could wash the thing in one side of twin tub, then extract in other all without removing anything.
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