Excel cell protection for formulas

I have a large Excel document which my staff are required to enter data into each week. There are various formulas in this document that I would like to protect to prevent the staff from accidentally deleting them or typing over them. The formulas are to calculate the data that they type in. I have tried using the "protect sheet" function where I have unprotected the cells in which the staff can enter the data. The only trouble is, because the formula cell is protected, it is not calculating the data when it is entered, it remains blank. Is this supposed to happen? How can I protect formulas so that they still change when the data is entered?