If you can't catch her alone then walk up to her when she's with her coworkers and tell her the treadmill/elliptical/etc. you're using is acting up and ask if she could come over and take a look at it for you. When she comes over be like "damn, of course now it's working when you come over. Thanks though. By the way, what's your name?" Just introduce yourself and leave it there. Don't try to get the number, this way you can sorta check her temperature. Then next time you're in the gym she knows your face so you can speak and make more small talk or shoot your shot forreal