An apparatus comprising: a rolling bearing including a first element and a second element, the first and second elements in tribological contact; ultrasound equipment to generate and detect ultrasound pulses; an analysis device comprising: an input, arranged to receive ultrasound data from the ultrasound equipment, the ultrasound data relating to a contact surface between the first and second elements in tribological contact, wherein the ultrasound equipment is arranged such that a propagation direction of the ultrasound has a greatest component perpendicular to the contact surface; an output, arranged to output data relating to at least one of a contact stress in the first element or a contact load applied to the first element; and a processing section (102, 105), wherein the processing section (102, 105) is arranged to: process the ultrasound data to determine ultrasound time-of-flight data, and derive the output data, wherein the ultrasound data relates to at least one ultrasound pulse, and the time-of-flight of the ultrasound is determined based on an envelope (825; 925) of the ultrasound pulse, wherein the processing to determine ultrasound time-of-flight data includes cross-correlating the determined envelope (825;925) with an envelope (815; 915) of a reference signal such that the determined time-of-flight of the ultrasound is independent of a phase shift dependent on a contact condition at the contact surface, and wherein the contact condition is changeable between a non-contact state and a contact state, load being applied to the first element via the contact surface in the contact state. a rolling bearing including a first element and a second element, the first and second elements in tribological contact; ultrasound equipment to generate and detect ultrasound pulses; an analysis device comprising: an input, arranged to receive ultrasound data from the ultrasound equipment, the ultrasound data relating to a contact surface between the first and second elements in tribological contact, wherein the ultrasound equipment is arranged such that a propagation direction of the ultrasound has a greatest component perpendicular to the contact surface; an output, arranged to output data relating to at least one of a contact stress in the first element or a contact load applied to the first element; and a processing section (102, 105), wherein the processing section (102, 105) is arranged to: process the ultrasound data to determine ultrasound time-of-flight data, and derive the output data, wherein the ultrasound data relates to at least one ultrasound pulse, and the time-of-flight of the ultrasound is determined based on an envelope (825; 925) of the ultrasound pulse, wherein the processing to determine ultrasound time-of-flight data includes cross-correlating the determined envelope (825;925) with an envelope (815; 915) of a reference signal such that the determined time-of-flight of the ultrasound is independent of a phase shift dependent on a contact condition at the contact surface, and wherein the contact condition is changeable between a non-contact state and a contact state, load being applied to the first element via the contact surface in the contact state. an input, arranged to receive ultrasound data from the ultrasound equipment, the ultrasound data relating to a contact surface between the first and second elements in tribological contact, wherein the ultrasound equipment is arranged such that a propagation direction of the ultrasound has a greatest component perpendicular to the contact surface; an output, arranged to output data relating to at least one of a contact stress in the first element or a contact load applied to the first element; and a processing section (102, 105), wherein the processing section (102, 105) is arranged to: process the ultrasound data to determine ultrasound time-of-flight data, and derive the output data, wherein the ultrasound data relates to at least one ultrasound pulse, and the time-of-flight of the ultrasound is determined based on an envelope (825; 925) of the ultrasound pulse, wherein the processing to determine ultrasound time-of-flight data includes cross-correlating the determined envelope (825;925) with an envelope (815; 915) of a reference signal such that the determined time-of-flight of the ultrasound is independent of a phase shift dependent on a contact condition at the contact surface, and wherein the contact condition is changeable between a non-contact state and a contact state, load being applied to the first element via the contact surface in the contact state.