The University of Michigan Health System's 4,000-member nurses union will go to work Friday without a new contract, though talks will continue through that day. Key differences remain to signing a new deal to replace the contract that expires Thursday, officials said. They include health insurance, retirement, overtime and paid time off usage, according to the Michigan Nurses Association, the collective bargaining representative for U-M's nurses."This employer has nothing to lose and everything to gain by taking their time in these negotiations," said John Karebian, MNA executive director, in a statement released by the group Wednesday. Members of the union include staff nurses and advanced practice nurses such as nurse practitioners, nurse anesthetists and certified nurse midwives. The average UMHS staff nurses pay rate is $33.74 per hour or $70,179 annually, according to the health system. The average UMHS benefits contribution to members of the nurses union is $15,852, including $8,217 for health insurance. The monthly health plan contribution paid by the average full-time MNA member is $127.47.