hourglass vs sandglass

hourglass and sandglass both are nouns.

Word	Noun	Adjective	Verb	Adverb
hourglass	Yes	No	No	No
sandglass	Yes	No	No	No

As nouns, sandglass is a hypernym of hourglass; that is, sandglass is a word with a broader meaning than hourglass:

hourglass: a sandglass that runs for sixty minutes
sandglass: timepiece in which the passage of time is indicated by the flow of sand from one transparent container to another through a narrow passage

hourglass (noun)	sandglass (noun)
a sandglass that runs for sixty minutes	timepiece in which the passage of time is indicated by the flow of sand from one transparent container to another through a narrow passage