Help - drowning in non-thesis related work from supervisors during PhD I’m not quite sure how to handle this situation and hoping you may be able to help!

I am over a year into my PhD and have supervisors who, although we don’t have a bad relationship from their point of view (they would likely say it is good), in my opinion I have a very formal, structured and slightly cold relationship (comparing to previous managers in other jobs). 

At the beginning of my PhD they were keen to get me involved in projects they have going on in their department that would help them analyse data and produce papers, I was told this was my way of saying thank you to the team (I assumed saying thank you for supervising me) and this would not gain me a name on their paper. Admit-ably at the start I was very keen to impress them and additionally thought getting stuck into some work for them could also help me get in the flow of what I might do for my own thesis. Over a year later and these projects continue and are only getting bigger each meeting we have. The tasks (and timeframe expected) mean I would need to spend on average 10-15 hours a week on these projects to get them anywhere near completion. I have noted to them in my last meeting this is a lot of work and I think I am unable to meet their desired deadline but this is not really taken on board. As our relationship is extremely formal I feel like I can’t really explain how overwhelmed I feel or how unfair I feel like this is. Its never been an option as to whether I want to or can take these projects on, they are expected to be done ASAP and I feel like I am being treated how you may a member of staff whose sole role it is to complete this additional work.

Having talked to friends and other PhD students they would call this unfair, toxic and flat out refuse to do it. However I am really mindful I want to keep in good faith with my supervisors, they are experts in the field I want to work in and there is no option of another supervisor to switch to without completely hindering my thesis and remainder of my PhD. I’d be better off quitting.

My idea would be to give it my best shot with finishing the majority of the project (even if it will be tough to do) , ensuring I am keeping minutes from our meetings which highlight that this is their project. Hoping that in future, if I've done a good chunk for them, they will let me away with not finishing acknowledging I have bigger priorities (my thesis). If this doesn’t work I would consider getting my university involved however I’d really rather not resort to this.

I know I’m being a wuss with this approach but I want to be diplomatic.
Any advice? Do you think it is normal for (some, I am aware not all) supervisors to do this?

p.s I don't care too much about not being mentioned in authorship for their papers, I'd much rather get on with my own project and have my thesis run smoothly. I just want to try and finish and be unscathed!
This is an unpleasent, but not uncommon situation. You say you "feel like I am being treated how you may a member of staff", and in fact many supervisors, particularly in the bio-medical world do treat their students exactly this way. For many students are the only mechanism to get any hands to do research. It is a bad thing, but it is common.

Firstly, no one should ever ask you to do work that won't get you an authorship, unless its group maintenance sort of work (organising the seminar rota, or making sure there is always milk in the fridge etc, monitoring everyones disk space usage). I know you say you don't care about this, but you should. Authorship is the currency of academia. It is how we are paid for our work. 

Secondly SOME amount of off-topic work helping others is part of the payment you make to them for training you. When the time comes, you might need help from someone else to achieve something your project requires, but you are not capable of. However, you need to be careful of this becoming too much. In the run of things, your 10-15 hours a week is probably on the edge of what might be considered too much. If you are work 50 hours a week, then thats 20%-30% of your time. Not nothing, but not leaving no time for your own work, probably just starting to interfere. 

Finally, expecting someone to do something immediately, when they are doing it as a favour is plain bad manners. 

You have two approaches. First, if your supervisors are being cold and formal, then be formal back: arrange a set of expectations that both sides are expected to uphold. This doesn't have to be confrontational, it can be approached from a neutral stand point. 

You time scales make it sound like you are in the UK. In which case, you should have an adviser - an academic in the department that is not scientifically related to your project. They are there for exactly this reason. The way this usually works here is that we have meetings with our advisees and if they report this sort of worry we go to their supervisor and say "I ask student X about their day-to-day and the answer made us worry that they are spending insufficient time on their primary project" - makes it sound like the advisor worked it out rather than you complained.